I am going to assume you want to know what a fork is. if not, ignore me.
If the sourcecode of a software is publicly available, someone can take that sourcecode and develop it into a different direction than the original software. In this case, a third-partt reddit app called Infinity is being developed into a Lemmy app by someone who is not the original dev.
Yes, 80 years ago. Todays Volkswagen has nothing to do with the original Volkswagen project beside the name and anyone who thinks todays Volkswagen is a Nazi company or any such nonsense is an absolute moron.