These are the default settings on my LG C9 for both modes. I personally use the Cinema mode. As you can see, no image enhancements are active on this mode, I only turn down Sharpness to 0. But when I do use the Cinema Home mode, I turn everything off, including this AI Brightness setting, resulting in all the settings being the same as the Cinema mode. If I then toggle between the 2 modes, the Cinema Home is a lot brighter. I guess that’s because it is meant to be used in rooms that aren’t completely dark, hence the “Home” suffix. In the clip at second 14, when I switch the mode you can see my phone adjusting the brightness of the recording. IRL there is a very noticeable drop in brightness.
I can highlight HDMI Mode with the mouse pointer but when I click on the center wheel while hovering over HDMI Mode nothing happens. I need to use the directional keys to navigate (one click on the right arrow) and highlight HDMI mode, then click on the center wheel and the HDMI info pops up. I don’t know why it’s not working on your C8.