Dietpi-VPN GATEWAY with Pivpn server and Pihole

Ok,
But i don’t understand because the portforwarding for vpn wireguard work
In a router:
Port In 58120 (wg0) → 192.168.10.5 (dietpi)
In the router i have the ddns active and exit with main IP (verified on site’s ddns provider)

And this work also with VPN Proton active on DietPi Transmission

This rule:

ip route add to default via 192.168.10.1 table isp
ip rule add iif lo sport 51820 to default lookup isp prio 15000

was modified also for transmission port? what is it for?