How does having multiple lemmy servers spread the load?

Basically, I have read several statements addressing this topic. For example:

"If my server gets too big I will just close registrations"

"Server X got too big, so they closed registrations to manage the load"

While I do understand that this can help for small servers which don't have a big number of external users. How does this help with big and popular servers? Don't they have to serve requests from external users using their resources? For example, I might self host a server just for my account but I read all my content from lemmy.world. Am I not using their bandwidth and their resources anyway?

Bonus question: Does federating with other servers increase the resource usage of my server? What kind of metadata/data do I have to store from each server I federate with?

Thanks!