HTTPS may be the official designation for the port, but it is the de facto standard port for TLS. Whatever you want to send over TLS, doesn’t really matter.

HTTPS is just HTTP served over TLS (originally SSL).

Step by step, if you were to analyze a web connection over port 443, you would see that the client first negotiates the TCP connection (via three-way handshake), then TLS, and it’s not till after TLS is established that HTTPS is negotiated.

In that way, it’s kinda wrong to say it’s the HTTPS port. It’s really, nowadays, the TLS port. HTTP is just one of many protocols that can ride on top of it, and when we do that, we call it HTTPS.