Hi there. I have a little question, maybe someone can answer.
If not, it isn’t a big deal.
I want to use these Icons
I think I need to get them from the OS, or is there somewhere else?
Regards
I make simple icons like that myself in Affinity Designer. Or you goggle “plus icon svg”.
Can’t you use a DesktopSegmentedButton with ‘Textured Square’ edges and a ‘+’ and ‘-’ as captions?
Most of the icons you see in MacOS will be available in the SF Symbols app.
Can he use these on his Windows applications ?
You know the answer. So why not instead offer suggestions for solutions? 
For example, I use a SegmentedControl whose segments are square and +,-, etc. as symbols for the various actions, if the project needs to look and work similarly across all platforms.
some options
fontawesome or symbol font
or Icons8 Pichon
I used icons8 for all my icons… brilliant!!