turtleboy

My first clue about how to use the Adjustment brush came from this YouTube video that I stumbled on by accident. (It is a little out of date so you can ignore the comments about it being a beta & lacking the brush size adjustments.)
 
Until I saw that I did not even realize that I could paint with the brush on the workspace to selectively refine the selection. Once I understood that, some experimentation showed me the basics, & searching Affinity Help on "selection brush" eventually led me to the "Refining pixel selection edges" help topic that explained the purpose of the matte, foreground, etc. brush settings.
 
Guided by that, more experimentation followed (which I'm still doing) to get a better feel for which settings work best for different photos. It isn't a "one size fits all" sort of thing, so don't be discouraged if you don't get the results you want on the first try or if you still have to use other tools to clean up a few areas that the refinement algorithms can't cope with.

My first choice would be to check the channels to see if there is one that will enable you to make a good selection. If you create a new grayscale layer from one of the channels, you can use the threshold adjustment to clear out parts you don't want, then make a selection from what's left. Then you can refine selection, or make a mask or whatever.
 
There is also the background eraser tool. I think that is really only good for quick checks, because I find the effect to be a bit harsh. Anyway, all you do is use the eye dropper to select foreground and background colors, then use the background eraser on the background near the edge of your subject. You may need to uncheck contiguous if there is hair or something where the green color shows through. You will need to play with the tolerance too, to avoid leaving green areas behind.

I was going to recommend the Flood Selection Brush as the first choice.  Thinking that would be give the best results.  But I am always amazed at how good the Affinity selection brush, with refine selection options is.
 
I haven't even begun to fine tune the refine and got really good results from just using the normal Affinity Selection Brush.
 
I'm no expert. Make your own decisions.  But my opinion says stick with the regular Affinity Selection Brush and Refine selection options.