The DLC levels (Schluesselschloss, Drachenfels, Karak Azgaraz, Stromdorf, Death on the Reik) can be accessed when invited by a Steam friend who owns the DLC (I have no experience with the console versions). I have recently seen many online posts claiming that ownership is only required for hosting.
Can solo players with only the base game play on the DLC maps? Do all lobbies show in the server browser? If not, what happens to vanilla-players when the server changes to a DLC map?