Your Proton password is not the encryption key for your data, not directly. Basically, your password is used to encrypt the actual encryption key inside your browser, and that encrypted encryption key is stored on Proton servers alongside your data. Proton can't access your data because they don't know your password which was used to encrypt the encryption key.
When you want to access your data, Proton servers sends the encrypted encryption key to your browser, and your browser decrypts it using the password you entered. Proton servers then send you your encrypted data, and your browser decrypts it using the decrypted encryption key and shows it to you. There's no point where Proton has enough information to decrypt your data. Your actual plaintext password never leaves your browser.
This is a simplified high-level overview of how it works, of course there's a lot more details to the actual implementation.