RPC function vs RoleAuthority check?

That completely depends on where you call “something”. If you do that in the BeginPlay, then yes, it might get called on every Server Instance of that Actor.
But if you have a single ServerRPC, it will only get called on the Server Version of that one Player who called the ServerRPC.

I can only recommend you to watch some more tutorial videos and maybe read the docs a bit more and, in the end, just play around with what you learned here.