I would like to make a world that follows all of our laws of physics except that every star has a habitable, earth-like planet in its Goldilocks Zone (I love that term). Each planet has a collection of plant and animal life that is almost identical to ours and a single humanoid immortal.
The immortals are just like us in all appearances and fitness. Some of them look male and some look female but due to their immortality they can't reproduce. They can be wounded, but heal very quickly (like regrowing a limb overnight kind of quickly). They cannot be killed but can get "stuck." Getting "stuck" happens when they get put in a situation that would kill a human but, for whatever reason, they can't heal from it. A good example of this is drowning, they can't breath so their brain gets shut down and if they can't float back to the surface on their own, they will be considered "stuck" until they can resurface.
Each immortal has his own planet and starts out with a basic understanding of their world. They know they can get "stuck," They know what a tree is, and they know how to walk and move with basic agility.
My question is not about how they could be immortal or even how to get them unstuck, but how their technology would likely progress. What kinds of innovations would a single individual care about? How much could one person do by themselves with an unlimited lifespan? Mainly I am looking to see what would be impossible for said immortals to achieve. I will probably ask a question later about the interactions of said immortals after they develop space travel, but let's focus on this for now.