Not the OS, but one of. We had our own community edition (the postmarketOS CE PinePhone) and since then there have been editions for Manjaro, Mobian and Plasma Mobile (and before us Ubuntu Touch).

Ubuntu Touch is it’s own beast. It’s Linux, but really closer to Android in how it works than a regular Linux distribution. It has a read-only filesystem and you install applications using it’s own Click format.

postmarketOS however is a true “regular” Linux distribution. The whole system can be written to, you have root rights by default and you update your system and install applications using a package manager (apk, don’t confuse it with Android’s apk format!).