I probably couldn't explain it all if I wanted to, and if I could I'd build some cool stuff with such knowledge.

That being said the relays do share into with other relays if I'm not mistaken, but the maintainers also filter and limit some of this to limit spam, I'm sure.

For example our relays are setup with one private and one public relay and our private relay sends all its events to our public relay from there the public relay takes all that data and shoots it to other relays to help spread the content across Nostr. to help make the content more easily seen.