>very few programs have own or additional "font-systems
Due to the Covid pandemic, I'm still away from my OS 9 computer, and am just going by memory, so can't actually test this myself right now, BUT:
The original method for adding fonts to the system was "Font/DA Mover". Even though no longer needed, I believe it continued to work in OS 9. I also recall that there was a little trick whereby if you held down the Option Key before selecting a destination, you could actually embed a specific font in a specific Application.
I was able to embed music notation fonts like Sonata, in music notation Applications, and remove them from the System, so that they didn't appear in word processing or page layout programs.
Using this method, it was also possible to add a special font that was embedded in an application. Some games had special fonts, and using this method it was possible to copy the font that only appeared in the game to the System, so you could also use it in a page layout application like QuarkXPress.