A few classic questions

I'm coming back after a several year break from retail to play some classic WoW and I don't fully understand how some aspects work if you could kindly answer a few questionsHow do the Classic expansions work? Can I still play vanilla if wotlk has just released or does it follow along with the expansions like original release? Is there separate server for vanilla classic and wotlk classic? And TBC won't exist outside of the wotlk classic?I heard something about TBC servers closing so don't understand how it works exactlyI'm also curious how PvP works if a realm is completely one sided? Does PvP matching go between realms or is it non existent on such a realm?Also curious about what realms would be best for UK/English speaking Europe?Appreciate it.