Boot ROM
The first code a chip runs after reset — burned in at the factory and impossible to update.
The boot ROM lives in mask-programmed memory on the die itself. Its job is to find and start the next stage: load a bootloader from flash, SD card or eMMC, or accept an image over USB or UART when nothing else is there. On systems with Secure Boot it is also the first link in the chain, since it is the only code that cannot have been swapped out.
Immutability is both the point and the problem. A boot ROM bug cannot be patched; it just lives on in every unit sold. Fusée Gelée in Nvidia's Tegra X1 (2018) made every early Nintendo Switch moddable, and checkm8 (2019) cracked open hundreds of millions of iPhones from the 4S to the X. On the friendlier side, STM32's built-in UART loader and the RP2040's USB mode mean those chips can never be fully bricked.