Since relays don't own user generated content, there is no need to "federate" client's generally rely on user-selected relay sets. The user chooses where they want to read/write events to/from.
That said, many of the "larger" relays do store events from other relays (federation if you prefer). Primal does, TheForrest does, nostr.land and so on. Nostr.land specifically has a purpose of aggregating notes from many other public relays, with spam filtering. It's a paid relay built for that purpose. Don't want that, use someone else.
Most users get to see 99% of notes from the current relay federation now, but it's also impossible to check those metrics.
Certain clients and signers store notes privately so if a relay ever decides to censor your notes you just publish to a different relay if they don't have your notes already.
Chances are if you use ANY of the popular paid relay providers, your going to get warnings on 3/4 write events that the other relays _already_ have the note published to the first. It's usually that quick...
Finally, relays "federate" by acting as clients themselves. Most relay software available already offers this as an option, may use it as a local cache for when on mobile and network/wifi is slow. Their local relay slowly pulls notes from other relays (or outbox) and caches those notes for when they load their client up. It's cache and the client dev didn't even have to write that functionality, it was transparent.
Finally, other's mentioned outbox, which has it's own set of issues as well, but it doesn't matter because a client developer can choose to give the user the option if they want. Going from federated, to peer-to-peer which was the intention.