A good example of an advanced bootloader is the UF2 bootloader used on CircuitPython capable boards. A UF2 bootloader is a way of programming microcontrollers.
Originally developed by Microsoft, it allows a microcontroller to enumerate as a mass storage device over USB (i.e. a USB drive). You can then drag-and-drop code onto the drive, and UF2 will re-program the microcontroller with your code.
This bootloader can work like an Arduino and accept serial uploads via the USB connection. Or it can emulate a disk drive that lets you copy Python programs to it.
An on-board RGB LED is used to provide diagnostic feedback in the form of steady or flashing patterns in different colors to indicate the state of the bootloading process.
UEFI