MacOS – How does macOS High Sierra generate bold weights for its system font

Apple's new system font, "San Francisco", has two versions – "Compact", for display on small screens like the Apple Watch, and a regular version for display on larger screens like macOS devices.

I am looking to replace the system font on my Mac from SF Regular to SF Compact. This is the procedure I'm using:
https://www.macissues.com/2014/11/21/how-to-change-the-default-system-font-in-mac-os-x/

Previously, the macOS font files in the /System/Library/Fonts folder included separate .ttf files for each font weight (bold, light, regular, etc.) On High Sierra, however, there is only one .ttf file containing a single font weight (regular).
The file is located here: /System/Library/Fonts/SFNSText.ttf. Using Fontforge, I replaced the alphanumeric glyphs of this font with those from the SF Compact file /System/Library/Fonts/SFCompactText-Regular.otf.

After restart, the following happens:

Any interface text that would normally be bold now fails to render.

For reference, with stock system font:

So, my question is: How does macOS display generate bold type instances of the system font?

What's interesting to note is that even when I only modify certain characters ("C", "r", "m"), the entire bold string ("Chrome") fails to render. This leads me to believe that the individual glyphs are not the issue, rather, something is being changed during Fontforge export. Fontforge does have a feature which allows for "Multiple Masters" – different weights and widths of a font contained in the same TTF file, but the original Apple-provided SF font (SFNSText.ttf) does not include any "Multiple Masters".

Any help would be greatly appreciated!

Edit: I am aware that there are also Display and Text variants of both fonts (SF and SF Compact). The menu bar uses the Text variant, so I am focusing on that one.