new to (official) wow question about wotlk servers

So i want to start in 2 days to play on wotlk classic fresh, EU, horde, mainly doing pve. But i want to be on the most populated server. Is that speculated to be the pve or pvp ?Also if i can ask what is the difference ? Like on pvp you are absolutely always pvp flagged and can attack and be attacked anytime anywhere ? -without it being turned/turnable offAnd if so, is the pve then going to be Only pve no pvp allowed (unless pvp zone/map/contested/etc), Or will it be: you cannot attack me, but if i attack you then you can attack me backThanks