Slightly confused, where’s the End Game?!

Evening all, just wondering when I get to the ‘end game’ proper?? I’m level 50, just beaten the big boss at the end of chapter 3 and now the game has thrown me back to the very start of the game?! (But harder difficulty!).

Sorry for the noobie question, but when exactly do I get to the ‘maps’ end game content?? (as I thought that would’ve followed the end of Chapter 3?). Thanks all 👍