End-to-end encrypted e-mail is an exceedingly difficult problem.

First, none of the envelope can be encrypted, sorry -- that's routing information, and it must be visible to all involved MTAs. The communications between MTAs can be encrypted with TLS, but the MTAs get to see the envelope.

Second, end-to-end key management is an O(N^2) problem unless you have introducers. Who shall be your introducers?

If the introduction problem was trivial to solve, we'd all be using PGP/whatever now. But it's not trivial at all.

Besides that, it's nice to have IMAP/whatever be able to search your e-mail. Which means your e-mail servers need to be able to see your e-mail. You can give up on this if you have your devices decrypt and index your e-mail. This is the only part of the problem that is "easy" -- and you can even encrypt e-mail as it comes in when it's not already encrypted.

How are they supposed to do that on the protocol level?

Protonmail allows you to add PGP key. Not sure if the user just sees 'garbage' data inside the email but it's entirely possible to send E2EE email already, just encrypt the contents of the message and send that across.

If the person has the key, they can decrypt it.

Do you really want to hand over your private PGP key to 3rd-party company? I never ever won't do that. If I will use PGP key for web email service, it is only when the service provider gives a way to communicate with my local machine so that the email text is SIGNED IN MY MACHINE and send it back to the email provider, then send to the recipients. For encryption, it can be done with public key of the recipients.

Eh, frankly while I Trust google a lot, I wouldn't trust any third party including Google for my end-to-end encryption needs.