I have thought of a sort of desert planet that has a atmosphere pretty similar to earth and with gravity a little lower which has little to no water source and most of it is empty dry wastelands. I also thought of a organism sort of resembles a camel that can produce water on its own to survive in such planet.
Is it plausible and if how?
You are generating new water molecules in your body right now.
You just did! And again! That assumes you are an air breathing biologic organism; no offense meant if not.
When we combine oxygen with food molecules and burn them for energy, the result is CO2 and H2O. This answer (yes it is mine which is how I could find it) goes into the chemistry. How could an organism store a massive amount of water?
There are organisms that produce the water they need this way. It is called metabolic water and these organisms don't need to drink.
Burning hydrocarbons like gasoline also makes water. That is why car exhaust is easy to see in the winter - it is full of water vapor. I had a scheme for Innocentive about generating water for soldiers in the desert by capturing it from their diesel vehicles. It did not win. :(
What your desert things need is something with hydrogen in it (any reduced carbon, any reduced nitrogen, possibly other things) and oxygen. The oxygen combines with the hydrogen to make water, and combines with the other stuff to make oxides. That is totally legit biology.
Fun fact: it seems camels emit less methane than cows or sheep.
You could make it so that your alien camel produces more methane rather than less. Let's say it produces as much as a cow, between 70 and 120kg / Earth year.
The alien camel could route that methane into a special organ, where symbiotic bacteria would oxidize it anaerobically. If your camel can also produce or eat nitrates, they can get three water molecules for every methane molecule. Otherwise they are capped at one H2O molecule per methane molecule.
The relevant reactions are:
Without nitrates:
CH4 + SO42- → HCO3− + HS− + H2O
With nitrates:
CH4 + 4NO3− → CO2 + 4NO2− + 2H2O
3CH4 + 8NO2− + 8H+ → 3CO2 + 4N2 + 10H2O
(That's a total of 12 molecules of water of output for four molecules of methane in the input).
That raises a whole can of worms about how the chemistry of those beings would be, but hey, if sci-fi will take giant worms that swim through sand and beings that can harvest sunlight from orbit, then sci-fi can take a camel with a water plant on its back.
Now, as for how much water that generates, let's do a rule of three. The atomic weight of hydrogen, carbon and oxygen, rounded to integers, are 1, 12 and 16, respectively. So:
$$\frac{H_2O}{CH_4} = \frac{18}{16} = 1.125$$
$$\frac{3H_2O}{CH_4} = \frac{54}{16} = 3.375$$
So for every kilogram of methane, your alien camel can produce either 1.125 kg of water or 3.375 kg, depending on how they can metabolize it.
Now, to see if that is enough, I did some quick googling and found this:
Camels can go up to seven months in the desert without drinking water. During such a time, they may lose nearly half of their body weight.(...) Very thirsty camels can drink up to 100 liters of water during a single visit to the well. (...) Camels are just very efficient at using water, and they’re well-adapted for dehydration.
100L of water for ~215 days means a camel needs 465 mL of water a day, on average.
If your alien camel produces as much methane as a cow, and can metabolize with nitrates, they could produce 70kg of methane a year, which would lead to 236.25 kg of water (which equals 236.25 liters in ideal conditions, gotta love the metric system). That's 647mL a day, more than enough to keep your alien camel going! The extra margin could be used comfortably for other activities besides its basal metabolism.
If your camel cannot use nitrates, then it will generate about 216mL of water per day. That's half what it needs. It will not be self-sufficient, but it will only need to drink half the amount of water it would need otherwise. In a desert environment where water is hard to come by, this may be a very positive adaptation.
Several plants are capable of absorbing humidity from the air, but they usually require high humidity.
You might imagine this creature to have organic water absorbers - like, say, silica gel beads on its skin. The beads absorb air humidity from under the creature's belly, and when they're loaded with water, the creature might eat them, recycling the silica.
Another possibility might be for the creature to have an internal organic absorption refrigerator. This inner organ of the creature would suck in air, freeze the humidity out of it, then exhale the dry air. It would require quite an uncommon metabolism.
Or it could do this by gathering foliage cut-outs and other debris in a small pit, then covering it with its transparent skin and expose to the Sun. This would condense the saturated water vapour out of the air (it is a known survival technique: the "solar still").
Dew you think this counts?
While actually making water from nothing is a little challenging, maybe no one notices your creature gathering water. In dry places, morning is a cool time of day, and dew can condense on surfaces. Perhaps your critters are gathering water from dew, but no one realizes that is what they're doing.
Desert animals take advantage of dew as a water source, some even using unique methods to do so. A company has developed a self-filling water bottle using a dew collector mechanism. Other dew catchers, like nets, spines, and modified beaks could be used to enhance these processes (this article uses high-tech devices, but based on biological designs) https://www.smithsonianmag.com/science-nature/five-wild-ways-get-drink-desert-180952845/
I'm assuming that you're assuming a breathable atmosphere (read: contains oxygen). In order to make water, you need both hydrogen and oxygen, which are potentially explosive when mixed together. (Water is the result of that explosive reaction, if it's set off.)
Multicellular organisms (and many of the unicellular ones) are actually already capable of synthesizing water; it's a byproduct of our regular metabolism of breaking starches into sugars, and sugars into carbon dioxide and water. (The process requires the addition of oxygen, which is why we need to breathe in oxygen - and breathe out carbon dioxide.)
When you eat and metabolize, you've brought in the "extra" hydrogen required to make water, captured in the starches or sugars. (We just happen to need more water than we make - since we use water to carry away many of the other waste products we generate.)
Your organism can eat something with hydrogen tied up in its chemistry, breathe in oxygen, and make its own water...
Except this is more or less normal earth biology.
Consider, like camels or cactuses, an organism which does a better job of holding on to the water it has?
All foods (more or less) contain water.
A random person on the internet claimed that a kangaroo rat can go its whole life without drinking (as you might guess from that, I haven't checked that). This does imply
It depends what you mean by "producing water on it's own" - certainly, it is exceedingly difficult to produce without a good source of hydrogen & oxygen.
I think, especially in a desert setting, that retaining water is more important than getting water - you only need to get as much water as you lose.
