Which projection is used when setting map coordinate system to a GCS?

[AlfredBaldenweck]

If I set the map coordinate system to a Geographic Coordinate system (e.g. WKID 4269), which projection actually gets used to draw the map?

[DanPatterson]

Set map properties | ArcGIS Pro documentation  under

Coordinate Systems—Use this tab to define and set the map's geographic or projected coordinate system. Scenes allow you to set a vertical coordinate system. You can also create a custom coordinate system based on the map or scene's region of interest. By default, the coordinate system is determined by the first dataset you add to the map or scene.

Are you finding a difference from this behaviour?  Is the map's coordinate system changing from what you set it after you add data in a different spatial reference?

I have always found the By default, statement to mean that if you don't set it, the coordinate system of the first layer added will set it.  Anything after that is projected on the fly to match it

[AlfredBaldenweck]

So, if I set my map's coordinate system to WKID 4269 (NAD 1983), it draws the map projected into some sort of projection.

What is it drawing?

My understanding is that it has to be projected in order to see the entire globe at once like this, so what's it using?

[AlfredBaldenweck]

This blog says it draws in a "pseudo Plate Carrée" projection, which is cool, but like, what is its WKID?

[AlfredBaldenweck]

Ok, using the search bar it's either 54001 or 53001, the difference being that one is sphere-based. 

Would love confirmation as to which was correct.

[DanPatterson]

NAD83 - EPSG:4269 nad 83 is the datum

now from the web

The Well-Known ID (WKID/EPSG) for the pseudo Plate Carrée projection is 4326, which corresponds to the standard WGS 84 geographic coordinate system displayed on a simple angular grid. [1, 2, 3]