While Painter 21 has some very good selection tools, it is missing one very useful one.
In Photoshop it is called a "quick selection tool" and Affinity Photo its called a "brush selection tool". They are unlike the P21 selection brush because they make intelligent selections based on the image being brushed . . . while P21 selection brush is all, and only, what is brushed (based on the brush stroke).
While P21 selection brush is useful and has capabilities those other programs do not have, P21 is missing a very useful smart selection tool. And there are intelligent masking tools too in other programs .... but having a smart selection; sensitive to image color/contrast/edges and by brush size and position, would be a much improved capability.
Try the Ps or AP selection tools and you will see quickly how much more they can add to P21 in terms of easy selections.
At this point, I sometimes export a P21 painting in process to Ps or AP, make the quick selection, export a mask, and import into P21....which is troublesome in itself; when it could be a simple tool use in P21.