Think of it like this; you have a local relay on your phone that you post to. It keeps a record of all your notes local and at the same time blasts the notes out to the broader Nostr network. One of those relays that it blasts to is also yours but it keeps a record of the data public for others to find. This could also go for your blossom media, you upload to your local relay as a “backup” of your blob, and then it mirrors to public blossom server that other people can find.