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?
I think with infinite time it would probably be possible in theory, but so much of todays technology relies on economies of scale that doing it the same way would not make a lot of sense for a single person.
So A smart person would probably create technology that looks very different from the stuff we are using today