I thought about this a few times but the flash storage is a bit sketchy for running a database, I am worried it can die. Tho android does have sqlite built in and works fine.
But the flash storage in a phone is going to be the eMMC variety which typically means a proper controller and TLC NAND. Not as robust probably as a "proper" SSD/NVME, but a lot more durable than most micro SD cards. And it'll have health indicators.