To follow up on PGP encrypted emails, it’s an encryption method that can be used on any email service, and is encrypted end to end. But it takes a bit of time and computer knowledge to properly setup. It has existed since the 90’s atleast.
To put it in really simple terms, protonmail is like VPN services, they make things hastle free with the setup already prepackaged, just login to the site or use the client, everything is done for you. Where PGP email encryption is like setting up your own VPN service manually.
It’s a very vague way of me explaining it, but I think it will describe the point.