How does macOS High Sierra generate bold weights for its system font?
If you go to Apple Fonts you can download the SF Pro or SF Compact Fonts. When I download and open the SF Pro I got 30 .otf files for Text and Display fonts. I installed these using Font Book. This installed SF Text and SF Display. SF Text contains 16 variations ranging from Regular to Bold and including Semi-Bold, Heavy Italics, and Text Light among others.