In simpler times, everyone that connected to the internet had an IP address. It’s 4 bytes, which means 4 digits between 0 and 255, so your IP might be 127.6.99.0. In these simpler times everyone could connect to everyone else, but if you want to run different services on your ip like a web server, email, etc, you had to do it on different ports. So if I connect to you on port 80, by convention that was web traffic. On port 21 was for file transfer. But that’s just convention. Any service can be on any port as long as you know which ip and which port to connect to. BitTorrent is no different. It listens on a port for connections from others.

It then got more complicated because people had multiple devices on their home internet connection and there aren’t enough ip addresses to give every device one. So then there was port forwarding (nat). All the devices share one IP address, and the router has rules for which port for incoming connections to forward to which internal device. Two devices can’t use the same port because there would be no way to know where to send the connection.

VPN is the same concept. You’re sharing an IP with that other people on one VPN server and if you want other people to connect to you you need a port forwarded just to you, like you would do on your router.

Very high level. I’ll try to answer if you have more specific questions