Android is linux based. Not sure that's what your asking tho.
Discussion
Is android open source?
Not bloody likely. If you are asking if there is an open source linux distro that you can boot onto a phone then I'm pretty sure the answer is no. Graphene OS might be the closest thing.
AOSP stands for Android Open Source Project: https://source.android.com
GrapheneOS is best, I use it myself but choice of phones is limited. The Google Pixel phones however are great hardware though and ultra secure. You won't regret to migrate.
LineageOS supports many phones: https://download.lineageos.org
And iode https://iode.tech/en/iodeos-installation is another privacy focused OS.