Bootloaders are typically loaded by the devices ROM or BIOS.
Device trees and more information about the Das U-Boot bootloader software.
A golden image is a firmware application with minimal support for firmware updating. If updating over-the-air (OTA), than this image must include basic wireless comms support. It is usually stored either in external flash or in a protected area of the microcontrollers internal memory. It is loaded by the bootloader if the main application is faulty, and in that sense, it is a backup application which is guaranteed to work.