Just an FYI, earlier this month the team spent some time polishing and publishing in source.android.com documentation about the flows the bootloader goes through in Android, specially true for stock Android like in Pixels phones or other devices based of recent AOSP versions.

Take a look at https://source.android.com/devices/bootloader/

This documentation includes the interaction between userspace and the bootloader such as the properties userspace expects when booting A/B devices, the whole A/B flow, the bootloader message in the misc partition (BCB), how they interact with the "recovery mode" in Android and much more.

Let us know if you have any feedback. deymo@