Just to clarify, your Bitcoin private key is stored in Proton, but it's E2EE (same as anything else in Proton, ProtonDrive, ProtonPass etc). The wallet is open source, you can find it here https://github.com/ProtonWallet
E2EE means you only have access to your data, it's stored encrypted at proton servers, even your PGP private key is stored at proton servers and it's encrypted with your proton password.