It uses Bonjour [1], which has an LGPL reimplementation named Avahi [2]

[1]: https://en.m.wikipedia.org/wiki/Bonjour_(software)

[2]: https://en.m.wikipedia.org/wiki/Avahi_(software)

For the sake of being picky, Bonjour is not a radio protocol (Layer 1), it’s a set of tools which enable zero config networking (Layer 2/3)

I agree, it's not a radio protocol and I wasn't trying to imply that it was. The person I was responding to posited it might be one and I pointed them to the protocol that's used without explicitly correcting them on that point.

Makes sense.

It can’t use just bonjour.

Apple devices can detect out of network devices as well.