The 100m winds in ERA5 are interpolated from the ERA5 model level winds. The interpolation procedure is described at:

http://www.umr-cnrm.fr/gmapdoc/spip.php?article157

Please read chapter 3: Vertical interpolations and extrapolations.

Feedback: C3S User Satisfaction Survey - CAMS User Satisfaction Survey

Web: C3S Help and Support - CAMS Help and Support