

You type the ip of the rpi on the router, so from an external call the router will forward it to the rpi. Or I don’t know what is your question.
Things may seem automagical in the networking scene, but you can config anything the way you want. Even in nginxproxymanager you can edit the underlying actual nginx configs with their full power. The automagic is just the default setting.
It seems you are missing some very basic knowledge, if you have questions like this. Watch/read some tutorials to get the basics, than ask specific questions.
This guy does the same thing as you: https://www.youtube.com/watch?v=yLduQiQXorc
This was like the 3rd result for searching for nginxproxymanager on yt.