Your client checks which relays people you follow write their notes to and reads from their "outboxes" 

 yana does a good job describing it here: https://github.com/Cyd0n1a/yana-Cyd0n1a/blob/master/GOSSIP.md 

 the only way nostr works as a censorship-resistant protocol -- formerly known as gossip.

the basic idea is instead of everybody writing to 3 or 4 or 10 "popular" relays each pubkey says "I'll write to relays X, Y and Z, you can find me there"

Now all a user needs to be able to make available is this very small event (which dedicated relays like Purple Pages focus on) that maps pubkeys to relays.

That way, if User A is censored by a vast majority of the relays, followers of that user can still continue to find their content without noticing that any type of censorship is going on.

(Imagine if all of Trump users could have continue to interact with his account when he moved to Truth social without even realizing that anything was happening)

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There's a lot more to outbox than this, but this is the basics.