In addition to the parent task, I have written comments for the font stacks in the wikimedia-ui-base.less file for quick reference and explorability.
Patch: wikimedia-ui-base.less
Related tasks
font-family-* variable name cleanup: T245136: Font family variables: use consistent names
Source of the documentation: T244790: Document WikimediaUI Base's font stacks, the research and reasons for each choice
Touched upon: T247166: Deprecation of `@font-family-sans` in WikimediaUI Base (tracking)