On the Wii for example, boot0 will load and check the hash of boot1 against the hash in OTP (One Time Programmable memory). Then, as long as the hashes match, it will load boot1. This means that boot1 cannot be changed in anyway, because boot0 and OTP can't be changed (not physically possible due to the types of memory used), so boot1 has to match the hash stored exactly (i.e. very high chance it is the exact same file). Then boot1 will check the signature of boot2, and if it passes this check, it will load it. (BootMii/boot2 worked due to exploit [trucha bug] in boot1; once this was fixed in newer Wiis, there was no way to reintroduce it, because boot1 can't be changed; this is also why Nintendo couldn't fix the bug and block BootMii/boot2 on already manufactured Wiis).