The majority of clients now (although not the ones with most users) are using the "outbox model" (also misleadingly known as "gossip model") which mean they will connect to any rogue relays they think they can find notes from -- so if I'm publishing to wss://pyramid.fiatjaf.com/ and you follow me then your client will connect to that even if that is not in your "relay list" (even the idea of a static "relay list" is an antipattern in my view).

This means anyone can actually run a relay and that relay will be useful and used.

How to find out which relays each person is publishing to is the hardest part and not straightforward, but in practice using NIP-65, NIP-05 relay lists and relay hints in events works well enough, and there are many client libraries that do it automatically.

You can reverse this pattern for other use cases, like mentioning people and replying to their posts in a way that they will see your stuff even if they don't follow you (NIP-65 specifies this) and do other variations for other use cases.