Hboot is the lowest level, which is equivalent to the bios of the computer and is responsible for the post-processing of the computer.

Recovery is equivalent to establishing an intermediate layer between hboot and android system. As the name implies, it is mainly used for system debugging and recovery. You can understand it as a system similar to windowsPE in PC.

SPL is equivalent to hboot+recovery. For example, G 1 brush machine is equivalent to downgrading SPL to obtain vulnerabilities, and then restoring the brush machine permission by replacing the signature.