What's the difference in wow realms offline

Difference WoW Realms / Server?

As you can see in the picture, WoW 1 says - Battle of Azeroth, the other 2 says nothing. What does that mean?

Are these different servers or how is it divided? Could someone explain that to me or something?

I got the passport (Azeroth) because I had a 1-2 month WoW phase.

Now there is an upgrade available (Shadowlands), if I get this now, will I get a new map or what is it like? For example, if I restart on WoW 2 or something without getting Shadowlands, do I have disadvantages or does this make no difference in the game?

Are these extensions from the game that I should get for all map accesses?

If I play on WoW 1 now, do I only play with the players who have the Azeroth expansion or do I see all players? How is that regulated? That kind of confuses me.

There are WoW 1, 2, 3 and when I start something I have to select server again. ???

Kind regards

NoobGamer4990 :)