Yes, it is just an app. A launcher replaces the home screen and installs like any other app, no custom operating system image needed; I researched the full custom build too and parked it, because you become the maintainer of your own phone every month for years. Mine, built in a day in August for one user, is a black screen with a text list of the apps I actually use, three cards for the weather, the agenda and my heart rate straight from the sensor over Bluetooth, and a terminal connected to the machine in my homelab. The phone keeps no photos and became a window into the one brain. There was friction, a first afternoon that cost sixty percent of the battery, but it does not feel like a step back.