When you do something like Reverse Proxy or Tailscale, your devices make an outbound connection to the Reverse proxy (or with Tailscale it goes to their auth/directory service) using UPnP.

UPnP is standard protocol these days, and how pretty much any communication or gaming app works. The port opening is performed dynamically by the router, the port number is different every time an outbound connection is made, and it’s ephemeral (both in the range and that the port closes after the session is complete). This isn’t something that’s typically blocked or disabled, as it would break all sorts of things.

en.m.wikipedia.org/wiki/Universal_Plug_and_Play

I may have misstated exactly how it works - I studied it when it was released, it became ubiquitous and always works, so I haven’t stayed current or reread anything for a while. It just works (and man has it saved me a ton of manual port config).