Used to level alts and mainly did pvp. I mained uhdk, ally and horde and an ally mm/surv hunter as main alt. not understanding almost everything in retail and been playing cata classic since I have an idea of what is going on there. how are uhdk currently for pvp? is pvp still going strong or no? I plan to lvl through remix to try and get an understanding of the game but I would also like to know changes to the different class skills and specialization of said classes. Basically I’m lost, truly lost with retail. any help or tips is much appreciated. also wondering if cata is populated, did some end game bgs and quite a few times had to play against the same faction wearing either the horde or ally mask for the bg. Horde dk, Ally hunter at 85.
I don’t DK, and I don’t PvP, but here’s what I can contribute.
First, give yourself a break. We see lots of people coming back from the Cata-WoD era, and they ALL seem to be Dazed and Confused by the changes, and half-remembering what it was like before, so you’re not alone. Given some time, people do adapt!
UHDK seems reasonably well represented in 3v3
Is pvp still going strong? Truth is, nothing is going strong right now. It’s the end of an expansion. Everything’s over. People are doing Remix to occupy themselves. No doubt everything will be going strong at the end of August and into September. 
I’m not sure that Remix will be the best way to get an understanding of Retail. Yes, the classes and builds are the same, but the stats are so distorted by gear choices and gems, and combat is so distorted by the gem procs that it doesn’t feel a lot like Retail gameplay. It’s meant to be fun, not authentic. But if you feel comfortable there, it might be a way to ease back into the game.
If I were you, I would go to Chromie in front of the Embassy in Orgrimmar or Stormwind, and ask her to send you to an expansion. You pick the expansion. There is something to be said for Warlords, Legion, or Battle for Azeroth. Just work your way through it - quest, queue for BGs, whatever you do. Rather than trying to get some kind of overview of the whole game, which is pretty complicated, just focus on levelling and getting comfortable with your class.
That will get you to 60 easily and in a pretty straight line, and then you can move on to 70 through Dragonflight.
As for Cata Classic, Ironforge Pro seems to show pretty healthy numbers, but the fact that the larger realms are near 100% Ally or near 100% Horde might explain why you are seeing the masks in the BGs: https://ironforge.pro/population/classic/ It’s probably more about faction saturation than a lack of population.
You can look up wowhead.com or icy-veins.com or YouTube to explain any things that are not clear, or, of course, you can ask here.
thank you for replying and the tips along with the links. much appreciated.