Is android open source?
Discussion
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.