ProtonVPN works great via Wiregurd on Linux as well just not through the GUI; you can download the configs and connect through terminal or other Wireguard client
ProtonVPN also provides Wireguard config files if you don't want to use their shitty python based GUI. Supports port forwarding aswell, althought it sucks to set up and requires to manually disable ipv6 support
It's when you open a publicly facing port and map (forward) it to a local port your machine. In this case, it's opened at the vpn provider's public gateway. Otherwise, it would typically be opened in your router instead.
You can then configure your torrent client to listen on that local port that the public port is forwarded to. I think generally the public and the local port are the same number when using VPN.
If you do that, then others have the ability to initiate a connection to you instead of only you being able to initiate the connection to somebody else.
When seeding/leeching to/from someone else, at least one of you needs a port open. So, if you always have one open, you allow yourself to connect to anyone on the network regardless if they have one open or not.
Sorry if I confused you more, I'm not that great at explaining.