It only works through the Proton Mail Bridge application, which is only available for desktop. That’s because Proton’s end-to-end encryption makes it impossible to access your emails while they are on Proton’s servers via IMAP. They would need to be decrypted on the server, but that would make the entire encryption pointless. The Proton Mail Bridge connects to the server, downloads the encrypted data, decrypts it locally on your PC and locally exposes an IMAP server, which contains your decrypted messages.