This kind of noise texture is very much in trend from material design to modern vector illustrations. Some call it grain texture, some call noise brush and some call it "salt and pepper" technique. Still for the record, here is an example. https://www.behance.net/gallery/55599139/Tornado-Chasers https://www.behance.net/gallery/66265897/Mark-Boardman-Carrefour
[The illustrations may not be vector illustrations but you got the idea.]
What it actually does is create a gradient of noise or spots and blotches to shape up the colour-values more definitively. This is pretty easy to do in Photoshop and Illustrator I believe. But for Affinity Designer, the options are scarce. And none of them quite yields the desired effect.
Paint the layer in Pixel persona with a grain brush.
Now either I couldn't use it well or something else is wrong but the pixel brushes give me very pixelated brush-marks (I tried both default and paid brushes).
The other option is to duplicate the layer, fill it with noise from color tab and then use the transparency tool to control the opacity.
This doesn't work very well because the noise is computer generated and it lacks the artistic impression that we see in wild (any modern flat illustration which uses the effect)
Is there any way I can make it work to look like the examples (painterly noise texture)?