What a mess of a website..
What does "read from global" mean? What is a "relay" and what is paid vs public? Why does FF suggest adding snort.social as an application for "web+nostr" links?
Messages are cryptographically signed, and users are identified by their public key. Users typically follow other users like they would on Twitter, but the client manages everything, so there is no dependency on any relay.
Messages are typically posted to many relays at once and clients typically read from many relays at once. Jack Dorsey's message is being displayed here because it was copied to a relay that this website has access to. The same message is accessible elsewhere.
"Global" means that you download all the public messages that a relay has, instead of just downloading messages posted by people you follow.