Question When does session start actually begin?

It seems that the origin for ALL coordinates for both rotation and position in session or world space are determined at launch of the application.
Does this occur
1 when you first see the Unity logo?
2 when the camera first appears and the scene is initializing ?
or
3 not until plane detection takes place?

 

All coordinates in ARFoundation are in 'Session Space'.
You can read more about Session Space here:
https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@4.1/manual/index.html
The Origin for Session Space is normally determined when your (first) AR-Scene loads.