I have AOE2 HD Edition on Steam, should I play the game by itself or are there other versions of the game?

Hello there, so I've been watching a fair bit of AOE2 on YouTube lately and it has inspired me to reinstall the game and get playing again.

I've mainly been watching T90 and in some of his videos, he says that this is AOE2 (X) version of the game. Should I play the game as it is on Steam or are there mods out there that the multiplayer community use to balance the game or something?

Also, I sometimes see civilisations in his videos that I don't remember from playing the game 15 years ago or civilisations that aren't on the Steam version, are these civilisations part of community patches? Thanks for reading