The way that relays and clients look up notes (and other stuff) is best described, strangely enough, by this video of a missile guidance system. It's hard to believe at first but if you replace "missile" with "note" you'll have a video that pretty much describes how nostr works.

https://video.nostr.build/a656a5ccd12575f7267399f5a9ebebc2abe974b33b527d1d63e9e93336a54b7d.mp4