Volla OS looks like an Android fork, so more like GrapheneOS than postmarketOS/etc.

When buying the phone you can choose to have it with Ubuntu Touch instead of Volla OS. There is also multi-boot.

Ubuntu Touch is still an Android-based OS (they used to have a variant that didn't rely on Android, but discontinued it).

Thanks for that precision, I didn't know about that fact. For those interested : https://wiki.ubuntu.com/Touch/ContainerArchitecture