Returning player, wtf is all this stuff?

Haven't played since the start of BFA, and I am overwhelmed. There's just so much shit everywhere now.

My max level characters from BFA are now level 45? Wtf

I can move the UI around? That's neat.

What are delves? Why do they have seasons?

What is renown? Is that different from reputation?

Pet battles and archeology aren't a thing anymore?

Cloth only drops if you're a tailor?

Professions are tied to the expansion areas?

There's like a million mounts in this game now.

Sky riding? What the hell is that? Why can I use that but not regular flying?

I feel like I've missed out on so much that getting acclimated is going to be a challenge.