How can you do this in Minecraft with commands?

How can you set in minecraft that players can randomly spawn in a certain area

I think this is only possible on servers as a plugin

You can put the Worldspawm there, but I don't know if that's suitable for your server. It depends on what kind of server you have.

You set the world spawn in the middle of the surface and set the respawn radius to the radius of your surface in the world settings.

Anarchy Server so yes I think you know what it is but if not:
There are absolutely no rules and you can hack etc.
a spawn breaks very quickly

I meant if you want to have several such spawns. Only one is clear, but to my knowledge, several are not possible with setworldspawn.

One is enough: 3