There's more to it, but the main difference from, say fediverse upload, is that the file hash becomes the file name. So a client can query the server indicated in the note for the file, but if it is not there, it can look for the same file on any other blossom server it knows of. Typically, the access control is different from the things you are used to. I.e. your haven relay on relay.inpc.cat will not accept files from anyone else than you, and that's why you have to sign an event for the blossom to accept the file.