One font file holds a whole design space. font-weight, font-stretch and font-variation-settings pick a location in it, and the outlines, advance widths and font-wide metrics all follow. Nothing is faked: the weight comes from the font's wght axis, not from thickening a regular face. In the PDF each location is embedded as an ordinary static font, because PDF cannot embed a variable one.
ABC font-weight: 100 | ABC font-weight: 400 | ABC font-weight: 700 | ABC font-weight: 900 |
ABC font-stretch: semi-condensed (75%) | ABC font-stretch: normal (100%) | ABC font-stretch: semi-expanded (125%) |
Any value on any axis, not only the ones the keywords reach, and it wins over font-weight and font-stretch:
ABC 'wght' 250 | ABC 'wght' 650, 'wdth' 110 | ABC font-weight: 100 overridden by 'wght' 900 |