Proton Mail contacts → Android system address book.
A GPL-3.0 Android app that signs in to a Proton Mail account, decrypts the user's contacts client-side, and exposes them to ContactsContract so they appear in the system Contacts app (and any other app that reads contacts) the same way WhatsApp or Telegram surface their own contact directories.
Proton Mail can access your local contacts app, but not sync backwards with it on Android. This means that you do not have cloud sync for contacts if you are not signed into Google, or if you are kn a degoogled ROM. This app would be a way to get around this. Add a new contact to either Proton Mail or your local Contacts app, and it gets backed up to Proton & synced with both apps.