I don’t think they do. They switch to very low power states, on big.little systems they power off half of the CPU cores. But some CPU cores, and all of the RAM

I don’t think they do. They switch to very low power states, on big.little systems they power off half of the CPU cores. But some CPU cores, and all of the RAM are still powered, the main OS doesn’t have other place to live. With different OS design it might be possible to power off more components of the phone when idle, while being able to wake up fast enough.