iphone XS go dim on its own

Try a restart, and see if that clears this: Restart your iPhone - Apple Support

Here are the common display-related settings: Adjust the screen brightness and color on iPhone - Apple Support

If this is not a setting, not something like an overheated device, and is not cleared by the restart, I'd wonder if there's a hardware problem. Check with Apple Support about that, using the Get Support link in the upper right of this webpage.

Not sure what you're asking about...

The backlight consumes power, and will dim and blank the display to avoid the associated power drain.

The time before the screen dims and then blanks is configurable in Settings.

The backlight can also adjust brightness based on ambient light. That too is configurable.

See Settings > Display & Brightness for most of the related settings, including True Tone and Night Shift where available.

Also see Settings > Accessibility > Display & Text Size.