is vertex data based on ellipsoid necessary?

I think cesium's vertex data are based on ellipsoid wgs84.

but, when google earth and other programs create VERTEX DATA,
are they really using ellispoid ?

i just became curious why they use the word "shperical mercator".
does It mean that they are using sphere when they calculate tile's vertex data?

sorry for my stupid question.

Cesium’s globe and all of the related computations are based on the WGS84 ellipsoid. This is particularly important for precise representation of objects at altitude since the geocentric surface normal (the sphere’s normal) is different than the geodetic surface normal (the ellipsoid’s normal).

For more info, see