ESL5
Port 443 is the default for encrypted web traffic. If you imagine ":443" added to every site you visit like https://adobe.com:443 that's what is happening behind the scenes. It gets intimidating to people, so the port is hidden. You specifying :8123 is simply overriding the default.
What the video does, (I'm guessing because I bugged off at the like/subscribe/service pitch and couldn't be bothered) is take the default port and forward it to your device so visitors don't have to type in :8123 to access your site. This means anyone can access it (or at least the login page) at https://[your.wan.ip.address]
That is for convenience, but also easier for riffraff to find. Note, security by obscurity is poor design and there are ways to reveal servers on different ports.