

The core of Android is open source and named AOSP for short, it is built on top of the Linux kernel which is also free and open source. However Google have made AOSP not usable by abandonning most of the free app in favor of their own proprietary apps such as Google Messenger, Goole Photos etc… That’s why community project like LineageOS builds their own apps to make them usable instead of the old deprecated ones. Brands like Samsung, OnePlus, etc… are not building from AOSP but rather paying a licence fee to Google so they can use the Google Services that are almost necessary to modern Android.
OP is running GrapheneOS which is a Free and Open Source Operating System based on AOSP.
GrapheneOS is a harden Android that gives you more control over your phone and its data. It’s not the only FOSS variant based on AOSP and works only on Google’s Pixel hardware as of today for security reasons. I’ve been running it for years now and I honestly like it very much.