Nostr: How do follows work across clients?

Short answer

A Nostr follow list is itself a signed event, defined by NIP-02. A compatible client can fetch it from relays and show the accounts you follow, so the list can travel with your key rather than one app account. The client must still find the list, and a relay might not retain it. Back up important relationships if they matter to you.

Sources

NIP-02: Follow lists.