Can you explain Endgame gameplay like I'm 10?

That feels like a dumb question but I'm totally new to Diablo and the genre in general. I'm nearing level 60, still going through the campaign, and wonder what endgame is all about.

Yes I've watched videos and read some stuff on it already but these are usually advices on builds. But... what I am finessing my build for exactly?

Is the goal of endgame to loot better and better equipment by slowly increasing difficulty? Will it be about doing all the dungeons and events and wait for new seasons to get more content or is there something else to it? What's the purpose of starting over with a new character (apart from having fun with classes)?

Loving the game so far though, I hope it will feel like a game I want to keep playing after the campaign is over.