After campaign you start doing rifts where you get greater rift keys.
In greater rifts you get legendary gems that you can upgrade after finishing a greater rift.
End game is to see how high you can finish a greater rift. Max is 150.
Do rifts, greater rifts and bounties to upgraded your gear. You want ancient and primal ancient gear so that you can enhanced it with gems to get higher main stat for more damage. But a normal piece with the right stats is better than a ancient one without it.
If you by any chance made a seasonal character, you can do your journey to get a free set for your class. After that focus on doing the altar of rites.
Sync supports comment jumping.