On Redmi 9 (MT6769), the standard EDL (Emergency Download Mode) is locked. Technicians use a modified scatter file loaded in "hot" mode to write a patched vbmeta and boot image, bypassing Google FRP without unlocking the bootloader.
When the phone has a corrupt preloader (no power, no USB detection), the only way to communicate with the CPU is via BROM. The "hot" method using test points is the last resort to write a clean preloader. mt6769 scatter file hot