Someone else mentioned it's in the settings. But AFAIK, graphene has pretty strong guarantees around no VPN leaks meanwhile Apple can't even promise that, they have holes. So it is a good feature. You just need to know how to occasionally stop it when it doesn't work for whatever reason.

Proton makes good products but the last thing I want to do is tie email data to VPN data. Separate it IMO. No account VPNs like mullvad or ivpn are great. You just have to worry about expirations.