The iOS secure boot chain system uses secure boot chain mechanism to provide security in the booting process. We have seen many rootkits and malware that infect at boot level. The iOS secure boot chain ensures that low-level software is not compromised and iOS is running on validated iDevice.
The following figure is the block diagram for an iOS secure boot chain:

Let's study iOS secure boot chain step by step, as follows:
Boot ROM
This is implicitly trusted
It is known as a hardware root of trust
This code is contained in the processor and cannot be updated or changed
This also contains the Apple root certificate with authentic public key and uses it to verify that the low-level boot loader is properly signed and has not been tampered before loading
Low-level boot loader
This is the lowest level of code that can be updated
It also verifies the signatures of firmware of iBoot before loading it
iBoot