As far as I understand, when you follow or post notes, they are categorized as 'Events' and are disseminated to your relays.

This implies that if you have X relays that are completely distinct from those of another person, they would not be able to see your notes.

However, there are certain relays that automatically fetch data from other (popular) relays, and services that distribute your notes to all the popular relays on your behalf.

One issue with Nostr imo is the centralization problem of popular relays, as individuals will need to use at least a few highly popular relays to garantee that all their notes can be retrieved by all users.

Perhaps someone with greater familiarity with Nostr can provide additional insight, i'm still newbie.