Yes to the routing, no to the port thing. A URL (i.e. qbittorrent.yourdomain.tld) is simply much easier to remember and work with than an IP:Port combination (i.e. 87.253.143.32:8080).
It also has a security benefit, because if you expose your server to the internet, you only have to open the http(s) ports of your webserver in your firewall and not the ports of the applications behind it. The webserver will do all the communication with your backend and then serve the information to the requester, so you have a buffer in-between.
Less open ports = less potential points of attack.
At least that’s how I understand it. I’m just a hobbyist, so if I got it wrong, feel free to correct me.
Frankly, if you want to use nginx as your reverse proxy and don’t want to get too deep into nginx configuration files and stuff, check out Nginx Proxy Manager. It’s a GUI frontend that automatically gets you SSL certificates for your subdomains, super useful.