Bookmarks on Nostr are a standard list (NIP-51, kind 10003), so they live on relays, not in one app. Any client that supports the bookmark list reads the same one, and some keep private bookmarks encrypted inside it too. One trap worth knowing, because we fixed it in NOSTRAS: that list is a single replaceable event, so a client that saves a new bookmark before it has loaded your existing list can overwrite the whole thing. NOSTRAS (https://app.nostras.app) waits for the list to load before allowing a change, and has a Bookmarks tab to read them later.