You are correct. It works without root by switching between your preset brightness to 100% by turning it on. It was hardware dependant if the hardware supports it. Flar2's Kernel's had the mode to leverage the harwares max capable brightness I believeI'm pretty sure that without root, you can't manually toggle HBM with a simple app. At least that was the case with previous Pixel phones, they all required root to force HBM.
There are some phone manufacturers that allow you to toggle HBM without root, but - at least up until the Pixel 5, to my knowledge - Google is not one of them.