Welcome back!
The game did squash the levels. Characters were squished to become roughly half of their old levels, so Cata/Mists characters that were 85-90 would have ended up in the 40s.
If you are coming back from the Cata/Mists era, there are some changes you should know about immediately, and several you will meet later, when you hit (the new) level 80 max.
Immediately, you should make your way to Chromie outside the Stormwind Embassy or Orgrimmar Embassy.
Chromie will let you choose an expansion to level through. Dragonflight/Dragon Isles is the one that Blizzard recommends fornew and returning players, and I agree that it is the safest bet for uncomplicated regular play.
Some resources you might find useful while trying to remember, and learning the new stuff
Wowhead com is your quick lookup source for items, quests, and most everything else
Icy-Veins com will tellyou all about building and playing your class
This YouTube video covers pretty much all the basics. It is massively long, but broken up into 1-2 minute chapters, so you can jump right to the thing you are interested in:
https://www.youtube.com/watch?v=nnzdeoSMkvM
And in general, anything in WoW is also found on YouTube. Just search there. Do be aware of the date of the video - with all the changes, some older vids may be out of date.
Do please ask again if you have any more questions!