For the sake of simplicity, the prototypical character of this sort will be Bob. He is as normal as normal gets.
The elongated form of my question: If a normal human from our world was transported to a world where magic exists but is only usable because biological processes make it possible, what would allow a normal human to be able to use magic despite not having those processes? (Clarification is in the following explanation.)
Assume a regular human from our world who CAN'T use magic. He's transported to a fantasy world with magic. He has average skills and knowledge that doesn't help in being able to use or learn magic in this world. He's NOT the only person isekai'd away. Whatever works for him should work for anyone else who wants to do the same.
The world is your standard medieval, high fantasy world. The world is effectively a combination of an MMO and a unique world in its own right. There are many, many different races, more than a few unique to my story, albeit some of them share similarities to other concepts out there.
Some people in this world can use their mana to effect magic. Manipulation of their magical energy is a biological process that some have developed access to. (These people are what you'd consider mages, spell-casters, etc.) Not everybody can do so, but only because not everybody has taken the time to learn how. It's a process all of these humans have access to, but cont everybody does so, just like how not everybody chooses to become a singer in a rock band from the 80s.
Everyone in this world is "low-level". While the higher level characters can do some special skills like alchemy, it's a rarity. Magic usage is limited and mostly relegated to the basics. (It's pronounced, "Leviosaaaa!") The "master alchemists" by this world's standards can barely make potions, but those basic potions are still a marked improvement over the salves and powders made by pharmacists. Whereas a master alchemist in the game could not only make extremely potent HP and Mana potions, a master alchemist in this world can barely make a healing potion, but it's the fact they succeed that makes them referred to as "masters". This means these otherworldly immigrants can NOT rely on the people of this world to create something to give them magic or to have advanced or complex equipment to do so.
Mana Biological Process Explanation
Mana is everywhere. Think of it like oxygen. If oxygen binds to hemoglobin, we can call the chemical (likely a protein much in the same vein as hemoglobin) in the body that mana binds to managlobin (because I am very creative). Bob does not have any managlobin, meaning his body cannot absorb and maintain a supply of mana for him to use. If he does manage to get a usable supply of mana from an outside source by some means, it would dissipate fairly quickly as it can't bind to him as is.
As for people being able to use magic, think of it like breathing. Everyone breathes, obviously, but not everybody can control their breathing well without practice and training. You can look at singers and athletes as a comparison for this. They control their breathing so that they can perform as they do. Have someone who doesn't know how to control their breathing do the same thing, and they will struggle if not outright fail. As a result, most people in this world don't know/use magic. It's a minority of people who either dabbled in it in their spare time, were trained specifically to use magic, or were born incredibly lucky meaning they're basically cheating at life. (You know the type.) While there would be enough people to teach the basics of magic, that doesn't solve the issue of the natural inability of these otherworlders to hold mana to use in their magic.
Mana is basically woven into the life of this world. So, by this world's standards, Bob isn't technically any more "alive" than a rock is. A golem at least has mana flowing into it to animate it, meaning a golem is considered to be more alive than Bob as far as their indication of life is concerned.
On the upside, because his body doesn't use mana at all, he's protected from the side effects of mana deficiency: headache, loss of consciousness, pain, and (if prolonged) death.
Additionally, Bob isn't the only one from "our" world who was sent to this world. Meaning, if any of these people want to learn magic, any process that works for Bob needs to be equally available to them. Solutions need to reflect this.
We could cheat by giving him an artifact or skill that also makes it so he can use magic despite not having mana, but that is cheating and cheating is wrong. Let's not cheat. Even though it may exist in the world, Bob would have no way of knowing that, no way of finding those items, and no way of getting to the items without major acts of god basically handing them over to him. Also, others wouldn't be able to replicate that since the artifact would be gone.
They could just relax and live an easy life without needing to know how to use magic, but that is a boring answer that doesn't actually solve the root of the question.
Since these people are not built for this world they're now in, being able to manipulate mana to use magic should be a challenge; success in learning and using magic needs to make sense and be limited due to this status.
Everything in this world that is living or animated has mana in it. It is this world's version of determining life. Just as we determine life by certain variable factors; this is no different. If they try to join the local adventurer's guild, they should be unable to formally join because the registration system won't be able to recognize them as "living".
When someone like Bob gets the ability to learn/use magic, I want it to be limited, but able to grow.
They also can't just be gifted "managlobin" by one of the more inherently magical races.
This question was edited in response to the question being put on hold. I hope by cleaning it up and removing story-specific details it better fits policy.
For the sake of making sure there can be a Correct Answer:
The following is a checklist for a correct answer using only things from my original Question. (Meaning it shouldn't invalidate any previously given answers if they were valid in the first place.)
- A correct answer will have something that ANY character from the "real world" would be able to do and gain benefit from.
- It needs to be something that is a challenge to attain but it can't be a matter of immense fortune (in the respect of money or luck) in order to be able to hold mana/use magic. While I could make it be something where a degree of money or luck is involved, it can't be something that comes across as deus ex machina or overly restrictive. The challenge very well could be something that simply takes a long time to occur, though. It doesn't have to be physically grueling.
- It cannot be something done exclusively through a rare artifact. As far as the people trapped in this world know, items from the game world don't necessarily exist in this world. There will be notable similarities between the worlds, but it'd be a leap in logic to say, "These places share similarities so there MUST be this powerful item somewhere in this world." Even if they do come to that conclusion, there'd be no way of finding this item without going on a Quest. (It's not that they don't exist, it's just that it'd be impossible to make use of for the average person.)
- It cannot be done through a Quest. Not to say a quest line is out of the question (kill 10 slimes, now kill 3 beavers, now take me to see my granddaughter in the next town over, now kill 25 giant spiders, now fetch my glasses which I left back at my house when we left), but assume going on a major capital-Q Quest to an undisclosed location is too prohibitive.
- It should not be something that could in turn make a person more or less capable of otherwise using magic. For example, a dietary reason. (Just an example.) It should only give the potential to use magic. If a person eats 50 of a fruit, they should not be inherently superior to somebody who eats 20 of the same fruit or 60 of a different fruit. While it can give some degree of boost and benefit, it shouldn't be that important that whoever has the most fruit is now god. Alternatively, if a native from that world refuses to ever eat that fruit, (still just an example,) they should not be inherently incapable of using magic if they so wished to learn. Use of magic for the natives should still be a skill derived from an evolved trait (like with good singing), but use of magic for those from "our" world should be something they can still gain.
- It should be something that could occur through a biological or pseudo-scientific process. Eating a substance, absorption of a chemical native and common to this world, and so forth are ways people have already recommended that are similar in nature of "ingest the necessary compound and hope the body can digest and use it." Alternatively, a recommendation of "emulating the effects of magic through distilling the compound from the blood" was another way. It should be a matter of Occam's Razor for the answer. While I'm fine with using a scientific method, there should be a reason why the characters would find that as the answer and be able to use it. In short:
- It should also be simple in its complexity. The answer shouldn't convoluted. The answer should be something accessible even if hidden. (Hidden in plain sight is valid.) This is sort of the reason why "naturally developed by eating the local cuisine where the compound is commonly available" is a better answer than "go through a long, complex process of SCIENCE! in order to synthesize a compound that can be mixed with other materials to emulate the effects of magic." Science can be an answer, but it should be a simpler form fitting of medieval science.
Most of the answers so far hit a couple of these boxes. One of which (the ingestion answer) is so far the best answer I have so far seen when combined with details given by other users in its comments. None of the answers so far hits every last one of these checkboxes, though. The whole "it can't be something that abuse makes one overpowered and lack of use makes someone underpowered" criteria being the check box that isn't being filled in the so far best answer. That said, I hope this checklist helps make the criteria more clear and to help re-open this post.