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.
I have no idea about this app.
That said, I use Tuta in part because they allow contact sharing on Android. It may be worth checking whether this app is using a similar approach to Tuta, as I trust that their method is secure.