WGS84

Geographic Coordinate System
A Geographic Coordinate System is a spherical coordinate system which uses longitude and latitude.

From What are geographic coordinate systems? by ArcGIS:

The World Geodetic System 1984 (WGS84) is an example of a Geographic Coordinate System.

Projected Coordinate System
A Projected Coordinate System is a planar coordinate system.

From What are Projected Coordinate Systems? by ArcGIS:

The State Plane Coordinate System (SPCS) is an example of a Projected Coordinate System

Datums
A Datum is an anchor point which ties two systems (geographic and projected) coordinate systems together.

From Datums by ArcGIS we read:

The North American Datum 1983 (NAD83) is an example of a datum.

NAD84 and State Plane Coordinate Systems
https://en.wikipedia.org/wiki/State_Plane_Coordinate_System

Lower down in that article:

The government says so as well:

https://catalog.data.gov/dataset/united-states-stateplane-zones-nad83

The Earth is Not Round!
[https://gis.utah.gov/nad83-and-webmercator-projections/ '''The Earth is Not Round! Utah, NAD83 and WebMercator Projections'''] By the Utah Automated Geographic Reference Center

Anchor Point Datum
It has been asked what the ellipsoid and center-of-earth anchor point that is associated with the NAD83 is referring to. If the NAD83 is composed of state plane maps with a flat XY coordinate system, why is there a reference to involvement of a globe model?

From the 'The Earth is Not Round!' article we read:

The ellipsoid and anchor point associated with NASD84 is necessary to interconnect with other spherical models, in order to connect correctly at the correct location and interchange data.

First it talks about the spherical model:

Note "real world."

Then it talks about the flat model:

The passage is speaking of an anchor point to connect the two types of systems together.

The accompanying image is the flat map with anchor point:



WGS84 Requires Flat Maps
WGS84 relies on flat projections to give out accurate data.

https://www.gpsworld.com/data-collection-of-wgs-84-information-or-is-it/

https://www.gpsworld.com/data-collection-of-wgs-84-information-or-is-it/

Take a look at the list of projections for the ARCGIS software:

ArcGIS 10.7.0 and ArcGIS Pro 2.3 Geographic and Vertical Transformation Tables

These are the world-wide transformation tables used with ARCGIS, of which NAD83 is one of them. These are all flat coordinate systems. It is taking data from flat systems, because as the "Earth Not Round!" article says, these flat systems that are more accurate and give out more accurate figures.

Spherical Coordinates Projected To a Plane
The California Coordinate System

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

West Virginia GIS Technical Center

We read that the geographic (round earth) coordinates can be projected onto a planar (flat earth) coordinate system to accurately display distances.

The reader should ponder how spherical coordinates projected onto a planar coordinate system could display data and distances accurately if the earth is a globe. Why should these systems require that? Are maps with spherical coordinates are not possible?

If the earth is a globe, the opposite should be true. Projecting spherical coordinates onto a plane should make data and distances more inaccurate. Not accurate.

Classified Coordinate Transformations
Of interest, some coordinate transformations are not publicly known.

http://desktop.arcgis.com/en/arcmap/latest/map/projections/choosing-an-appropriate-transformation.htm

Flat Maps as Standard
We find several sources which describe that flat maps are the standard in GIS work.

From https://www.e-education.psu.edu/geog862/book/export/html/1644 we read:

Another comment: https://www.pobonline.com/articles/86585-geodetic-surveying-made-plain

We see that flat maps are the standard.