Check this post - Paint selection (test)
Posted a request for this a few days ago, and then realised that, it’s not a good idea to modify selection mask. I make this plugin to use a paint layer for selection. It works (though not perfectly).
I still want krita implement this as a default tool.
This is the video from that post showing how the tool works -
So you paint with a paint brush and it turns what you painted into a selection. Selection Mask and editing an exisiting mask by painting on it is a different thing. Just like you draw a rectangle to get a rectangle selection immediately.