The Mac boot process is completely unlocked, at least on Intel Macs.
On Modern Macs, the process is somewhat convoluted, but you are able to boot into a custom compiled boot loader / operating system while secure boot is enabled. It just needs a few minor hoops to sign the boot loader - steps that would be difficult to social engineer around but perfectly reasonable to do them intentionally if installing an alternate operating system is your thing.
iPhone is, of course, a different story. Hopefully that changes some day. The CPU and boot process is the same as a Mac, so there’s no reason it couldn’t be unlocked. Might require government intervention though.