I don’t know hardware/systems like this. I’m an abstract thinker. I like to break down and distil complicated subjects to a level I intuitively understand.

Secure boot uses a Trusted Protection Module (TPM) chip. It is usually a dedicated chip, although there is a sketchy way it can be implemented in software. The idea is that the TPM chip is not connected internally to your computer at all. The chip can issue a new key that is only hashed with an internal key on the chip. There is never a point where that internal key on the chip is present in your computer. If any key present in your computer is generated from a TPM module, there is no way for anyone to swap, modify, or access the private key pair inside the TPM.

Secure Boot is using this key to check that the bootloader and all kernel code hashes against the TPM secret key. Running SB with a TPM keyset is essentially saying no part of the chain is trusted, so we’re making an extra sandboxed place that we can trust and making it exist in a space with no possible connections in software. There are possible exploits in an electronics lab with physical access to the hardware, but that is well outside of the average person’s reasonable security model.

If anyone modifies the bootloader binary, it is extremely difficult to detect after it is compromised.