Classic Coming in from legion/BoA, what's changed?

I got to level 110 or 115, so with shadowland squish my DH is lvl 50, druid monk lock and sham all are 45, and rogue mage hunt are 40.. what are some major changes I can expect , coming from legion? I hear pvp gearing has changed massively and pre req for pvp are grindy. I just don't know where to start really. And haven't fully immersed myself in the game since probly WoD/ somewhat in legion. Thanks yall. Just wanted some pointers on a few things to expect. And since I hear it's not very alt friendly, I'm still kinda trying to figure out which character I'll have the most fun with in pvp via /r/wow https://ift.tt/3sYMhMQ