I think that DM regicide Empire Wars King of the hill are different game modes. They are options you can select in lobbies.
however Nomad and Mega Random are maps. You cannot select to do Nomad as a game mode in a lobby to play on any map (though that would be great!). You have to pick those maps that have nomad starts. As such they aren’t any different from maps like arena or fortress. In those you start with more buildings, in nomad you start with less, in mega random it could be more it could be less.