Or you defer bootstrap until you have some channel on which you can piggyback. For example if you download some file then you need to exchange some content-ide

Or you defer bootstrap until you have some channel on which you can piggyback. For example if you download some file then you need to exchange some content-identifying token anyway, a magnet link in bittorrent. That link can also contain a bunch of nodes the source peer knows, preferably the longest-lived ones it knows. Depending on where it's used you could also turn this into a QR-code, NFC peer or whatever. Practically speaking an implementation will already contain a bootstrap list or might download it from somewhere else on first start. But that's just for convenience.