I am newcomer and I don’t know where are brightness control
That depends - if your display supports it - that it is mostly laptops - then the fn key with the relevant function key usually does the trick.
I have only found one window manager - compiz - which allows doing the same thing with other than laptop displays - but since compiz is not wildly in use on Manjaro changing to it you are much on your own and as you put it - newcomer - I will not recommend experiments.
I have not key combination for brightness on my notebook with manjaro.
So it is looks like manjaro is only for sleepless at night.
Not for sunny days
Please tell us more about the make and model of your computer.
There is the
xbacklight -inc 10 increases the backlight by 10% points.
-dec decreases it by 10 points.
That sounds strange - nearly every laptop I have been in touch with over the past two decades had a second function attached to any F-key which was activated by pressing a special fn key.
Which F-keys are used for brightness varies but the laptops built-in keyboard usually has this.
@mithrial’s suggestion is actually a good advise - at lest you can try that.
As @TheHappyHobbit suggests - it would be nice to know the make and model of your device.
I just ran across this on XFCE and Power Management.
The above command showed “Has brightness panel”. I didn’t see anything about brightness in Settings > Power Management, but according to this entry this is where it is suppose to be.
Here’s the doc for KDE power management, which control brightness.