no that is not how it works

nostr notes aka posts are signed self provable things, they can come from anywhere, you could stick them on a usb stick and hand them around, make a p2p network for them

as is now we use servers called relays, we usually publish our notes to a few for redundancy but also because we dont all use the same servers, i use 30 which is a bit much, others use 6, relays also exchange notes with each other, i can also save my notes or others notes and i can reupload them to any server

ie notes can come from anywhere

there is a bit to work out on making the whole thing more efficient and finding things better but it generally works well

a self hosted fedi instance may go down and can be blocked, how does it work in china for example? nostr is doing well in china