Router gets the public IP. Login to it, find port forwarding option. You’ll pick a public port. IE 443 and forward it to a local IP:port combo, IE 192.168.0.101:443.

Then you can pick another public port and forward it to a different private IP:port combo.

If you want a subdomain, you forward one port to one host and have it do the work. IE configure Nginx to do whatever you want.