A bootloader is a vendor-proprietary image responsible for bringing up the kernel on a device. It guards the device state and is responsible for initializing the Trusted Execution Environment (TEE) and binding its root of trust.
The bootloader is comprised of many things including splash screen. To
start boot, the bootloader may directly flash a new image into an appropriate
partition or optionally use recovery to start the reflashing
process that will match how it is done for OTA. Some device manufacturers create
multi-part bootloaders and then combine them into a single bootloader.img file.
At flash time, the bootloader extracts the individual bootloaders and flashes
them all.
Most importantly, the bootloader verifies the integrity of the boot and recovery partitions before moving execution to the kernel and displays the warnings specified in the section Boot state.