WGS84

The Earth is Not Round! Utah, NAD83 and WebMercator Projections
By the Utah Automated Geographic Reference Center

It says that "The UTM NAD83 projection uses the GRS80 ellipsoid and a center-of-the-earth anchor point as its datum,"

“ The State Plane Coordinate System was established to provide a means for transferring the geodetic positions of monumented points to plane coordinates that would permit the use of these monuments in plane surveying over relatively large areas without introducing significant error.

A plane-rectangular coordinate system is by definition a flat surface. Geodetic positions on the curved surface of the earth must be “projected” to their corresponding plane coordinate positions. Projecting the curved surface onto a plane requires some form of deformation. Imagine the stretching and tearing necessary to flatten a piece of orange peel. In California the Lambert Conformal map projection is used to transform the geodetic positions of latitude and longitude into the y (Northing) and x (Easting) coordinates of the CCS83. ”

We read that the spherical coordinates are projected to the plane coordinates. Not the other way around.