There's also Apple DFU mode to restore OS with help from immutable ROM and external device, without depending on installed OS or mutable "Recovery OS" partition, https://theapplewiki.com/wiki/DFU_Mode & https://support.apple.com/en-us/108900

> DFU or Device Firmware Upgrade mode allows all devices to be restored from any state. It is essentially a mode where the BootROM can accept iBSS. DFU is part of the SecureROM which is burned into the hardware, so it cannot be removed.