I wonder how is the modal scale found and why does working this scale? I mean as example the dominant modal scales which are different versions of mixolydian. But why?? Why is there used Modal scales and how can I find the right one for the different chords?

This makes absolutely no sense whatsoever, can you clarify?
Perhaps my post on Scales, Modes and Chords might be helpful.

JJF, I always enjoy your posts as you know...I'm just wondering, do you have a blog or music theory website of your own?

never heard of it.

I think you are confused. You are assuming that, since Mixolydian is arrived at as though the fifth mode of 'Major', and the 'V' chord in harmonic usuage is called 'dominant', eg., 'G mixolydian' is the thing you want on a 'G chord', the 'dominant' in C major?

The dominant function here, of "V chord = G" belongs strictly to *C* so the term 'G mixolydian' doesn't have any real meaning in that case. IE: this 'G mixolydian' is still a C major scale. The right scale for C major is C major. 'G Mixolydian' has meaning when the tonic is *G*.

I don't know what 'dominant modal' is supposed to mean. If that term has currency it's going to arise out of confounding the fifth degree of major (as the way to 'Mixolydian') with the term 'dominant'.

Maybe you heard some jazzer talk about scales that resemble Mixolydian. The reason for those are going to tend to follow alterations of say the dominant seventh chord. It's going to be clearer to make a distinction 'scales' vs 'mode' here.

as point of reference, you may notice that any seven note row is going to be coincidental with seven other rows that share the same notes but reordered. but the meaning of them as a mode belongs with the first note of the row being the tonic.

if you want to apply chords to the thing as a mode, the first note of the mode accords with the I chord.

vs. thinking that with eg., C as tonic there are seven useful modes incl 'G mixolydian'. These only owe to coincidence. There is no reason for such naming.

To try and answer your question why, out of what I imagine is your question at any rate: Say you have a scale - G Ab B C D E F that someone called 'G mixolydian flat two' and thought to describe it as a 'dominant modal scale'.
The thinking is they are dealing with an extant G7b9 type chord (G B F Ab) going with a scale that fits. This isn't any 'modal' however, which is a confusion of two things that aren't the same.

applying *modes* to extant chords is upside down. The chord progression may very well be major or otherwise unsuitable for the mode. Say you have a progression in C Major. The modal terms eg., 'D Dorian' have no real meaning, the scale is C Major, there is no reason to call it the other thing, and the point of eg., 'D Dorian' is the seven tones in it relate to D in the first place. So if you do, for whatever musical reason want chords and a mode to happen together, the chords have to respect the mode. In general, sticking to a couple of chords rather than trying to be cute is preferable, a couple of chords that bring out the character, via the character tones of that mode.