it's nothing complicated, if you want an easy to launch shortcut you can look into Termux Widgets and write a shellscript

oh..

Def is

Ah yeah, it's just a translucent overlay slapped on top of everything. It's the fallback option when nothing else works.

Lots of apps that advertise lowering screen brightness below 0% just do that, so I thought that's what you mean.

There is also a way to properly filter the colors, which doesn't discolor things as badly, but I believe you need root for that, or for it to be built into the OS, like the "Extra Dim" option that various ROMs have.

I guess, there might also be ROMs where the manufacturer decided to limit how low you can set the brightness via the UI, even though the hardware would accept lower values. In that case, yeah, try what @hexagonwin@lemmy.today posted with writing to that file via ADB.
Maybe you can also do that via Termux somehow, if you've got root on your device.