Alexey Gamov

Hello. I need help. Plz tell me someone how to make a gulshat in affinity designer like on the screen? (something like waves formed by many curve lines)...

I don’t have time to write down all of the steps fully but this GIF might give you the clues you need.
(The end result in the GIF doesn’t look very pleasing but that’s just because I did it quickly, you’ll need to do it better to get a better result, but all of the steps are there.)

gdenby’s technique can produce some lovely results without much work.
Well worth experimenting with.
My second example uses an Acrylic brush and the Subtract bend mode.
My third example uses all kinds or semi-randomly chosen effects (not particularly useful but interesting).
My fourth example uses a bitmap stroke fill, a Gaussian Blue effect and an Add blend mode.

As Callum says, this sort of thing is only possible in Designer if you do it manually.
My top attached example was created by repeatedly duplicating a curve, giving it a larger stroke width, adding a pressure curve, expanding the stroke, breaking the resultant shape at the ends, resetting the stroke and fill colours and then removing the unused portion of the curve.
My bottom attached example was produced by duplicating the curve, resizing it in the vertical axis and re-aligning it with the original.
Neither is what you want but they might give you some ideas.