Correct. To be pedantic, there is a built-in 16kB ROM starting at address 0x00000000 with RPi's open-source BootROM burned into it at the time the silicon is m

Correct. To be pedantic, there is a built-in 16kB ROM starting at address 0x00000000 with RPi's open-source BootROM burned into it at the time the silicon is manufactured. There's no publicly-known way to write to this. It's technically "non-volatile storage", but it's not useful to anyone other than RaspberryPi/TSMC.