White space characters calculation in InDesign

Does anyone know, how InDesign calculates the white space characters (emspace, enspace, fourperemspace, hairspace, nbspace, thinspace, threeperemspace), if these width values are not defined in glyphs?

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I would also like to know, I’ve a font in which all of these remain the same value in InDesign.

There is a difference between TTF and CFF. But I do think it takes the font size as basis.