"It depends" as not all the names you listed as examples use the same technology, but in general "UPNP is more useful for thins which need an incoming connectio

"It depends" as not all the names you listed as examples use the same technology, but in general "UPNP is more useful for thins which need an incoming connection" (kinda sorta). This might be, say, a bittorrent client needing to allow other clients in on a port to share the file... sharing. To share. :) If you understand how Active vs Passive FTP works and how the incoming connections might need to be tracked (nf_conntrack for Linux folks), UPNP is more like that - apps which handle bi-directional conversations with the outside world beyond your router. Airdrop uses an ad-hoc WiFi network (peer-to-peer) with TLS, as does I think (Android) Beam. If I'm not mistaken some other devices in this area (Chromecast, Roku, etc.) use similar techniques, and sometimes leverage bluetooth ad-hoc networks. Discovery services like printers and fileshares tend to use (I'm assuming you're macOS) Bonjour (Rendezvous, renamed awhile back), which is sort of like an ad-hoc multicast (mDNS) solution if I understand it. On Windows it would use something like Netbios - conceptually the same. I just set a static IP on my wifi printer and call it a day, it's trivial stuff being a printer.