What counts as endgame content?

I've recently unlocked the empowered monoliths and am doing fairly ok with the base 100 corruption as a fire/ignite runemaster. But whenever I look up builds (on lastepochtools) they rarely mention the corruption ceiling (or even just a comfortable ballpark) the build can run.

When I look up videos they rarely mention it as well. Some will say what feels like low corruption amounts like 300-500. But then I see some videos where they play builds at 1.5k corruption and more which is bound to be way harder than those 300 builds.

So what exactly is end game in LE?