Let’s say we left one single very smart guy (not necessarily with the knowledge: they may be able to understand hard stuff when taught it, but not know it already) alone on a copy of the earth. That person is also immortal. Could that person, by themselves, gain back all knowledge, maybe also experimental, or even surpass that is already available to us right now, before the planet gets inevitably engulfed by a sun turning red giant?
No. Think of how big the category of technology involved in keeping the human body alive. Ok our immortal wants to recreate the pacemaker but there is no one else but them around. How would that work exactly? Presumably as an immortal they wouldn’t need one. So how would they test it?
Then you add in things that only make sense if there are multiple people in the world. You don’t exactly need passenger side airbags if there are no passengers. Tandem bicycles would be strange for this lone immortal to make. Everything involved in childcare from the simple (how to swaddle a baby) to the surprisingly complicated (breast milk pumps) to the obvious (diapers).