>If I told my mom to install some app and we'd connect to each other using some random IDs, we'd probably still be using SMS

This is surprising to me, since it was my mother's generation that got me into Skype (where you have to manually add the other person by their username), almost twenty years ago.

A Signal support claims "The Signal service does not have any knowledge of your contacts."

Can someone explain how this is possible? How does my phone know when to notify me that a friend has joined signal?

Edit: apparently it's "private contact discovery", I need to give this a proper read later.

https://signal.org/blog/private-contact-discovery/

Yes, their blog posts are really good. Read about the sealed sender, too! And then realise that even with sealed sender, if you send from your home IP, then you are identifiable. So you should send from a reasonably anonymous IP (w.r.t. your threat model)

I will never get over the fact that the Nintendo Switch operated on random character strings to find friends rather than usernames or something more straightforward for normal users. But the fact that they managed it does mean it's not so large a barrier.

I guess it works because people think of it as if they were phone numbers, plus it removes the barrier of having to choose a username. See also the now defunct ICQ which was fairly popular even outside of tech circles.