If it matters, I'm running Debian 10 with "Plasma by KDE" (Kernel: 4.19.0-18-amd64)
(I dual boot with Windows 10)

I'm running a dual monitor setup, with HDMI to Samsung Smart TV (un32j5205af) and display port to a normal desktop monitor. I'm also using a GeForce 1070 if that matters.

This may sound like I'm asking an unrelated question about a TV, but please hear me out.

I'm curious if Windows sends a signal through HDMI saying "I'm an OS, don't auto adjust backlight" and if the same can be done with Linux or if there is some driver out there?

When I use Linux, if I open some dark/black window and increase its size to about 75% of TV screen, the TV will automatically start dimming its backlight. As I increase the window to 100%, the backlight gets darker and darker. (This does not happen on the desktop monitor)
I've went through EVERY single setting in the TV menu and nothing seems to prevent this(I've also tried Google which recommends looking at Eco settings), except one setting which enables dual input sources (Where I see main source full screen and a small overlay of whatever second source I want) and with this enabled setting, the backlight will no longer auto adjust.

At this point, I thought - "Well, it's a feature on the TV. Nothing I can do on Linux to change this"
However, when I run Windows 10, this auto backlight dim feature does not occur.

So I guess back to my first question: I'm curious if Windows sends a signal through HDMI saying "I'm an OS, don't auto adjust backlight" and if the same can be done with Linux or if there is some driver out there?

Reasons I believe the auto dim is from TV:
1. It only happens on the TV, not the desktop monitor
2. It doesn't happen when I enable the TV's dual-source display.
3. I ran a screen recorder during the dim test, and the playback didn't show any dimming.

Does your TV have something called "Dynamic contrast" or equivalent. Turn it off. I have a TV that will do that hooked to a PC with HDMI.

In addition to teckk's suggestion, look for 'ECO Mode' or similar - I have a Samsung monitor (not a TV) with this (Samsung specifically calls it 'ECO'), and it's just an ambient light sensor that tries to adjust the backlight versus how bright the room is, which can also cause this (usually in dimly lit rooms - during the day it seems to make a decent showing of itself against sunlight, but at night it can get a little weird); I believe Windows has a 'driver' of sorts that can interact with the monitor if you also connect the USB connection, but I didn't bother trying that out (but that may or may not be related here - do you have a USB connection here?).

I appreciate your answers and replies!

In the end, it was me, I missed something. I'll explain more of what I tried then what it was.

I did check Eco - "Eco Solution" which only has 3 items for me: "Energy Saving", "No Signal Power Off", "Auto Power Off" but none of these were it.

I also have tried setting "Dynamic Contrast" But didn't stop the auto dim.

I even learned about the "Secret Service Menu" (For my model it was "Power Off TV then press: Mute + 1 + 8 + 2 + Power")
And read about how to do the 0000 trick on "Advanced" to unlock even more secret stuff and to eventually look for some "2D_Picture" menu with some PWM settings.
I only had a "3D_VIDEO" menu and couldn't find any PWM settings anywhere.

After reading your comments I decided to try the regular settings once more.
Like I mentioned, I'm pretty sure I went through all the settings, all the "Picture Mode"s, but I prolly didn't test the differences thoroughly. It was "Picture Mode".
I've always had it set to "Natural". But read to change it to "Movie". After doing that, the backlight no longer automatically adjusted no matter what colors were on the screen!