Reddit is one website where everything lives. The website is made of many different parts that all run on computers that are owned/controlled by Reddit.
Lemmy is multiple sites where everything lives. Lemmy is made of many different parts that are run on different computers by different people.
These parts all speak the same language so for someone using Lemmy, it feels like it’s all just one website.
It isn’t run by them since the platform is open source. They run Lemmy.ml but other instances are free of their influence.