Robocopy /Purge /Move - at what point/how will destination folders/files be deleted?

I have read both At what point does Robocopy overwrite a file? and At what point does Robocopy overwrite a file?, and neither seem to address my question or I'm totally clueless. Example(s):

• Parent-Dir: Zip Code
• Multiple sub-dir(each day):yyyymmdd
• Sample filename:1_IPC1_NAPA_20240202001334.mp4
  (there can be an average of 5,000 MP4 files per folder)
  
  

With both the src and dest having less than 500GB of free space, I run:

Robocopy A:\Parent_Dir B:\Parent_Dir /PURGE /MOVE

• I'm not clear\sure if that command will /PURGE as needed, all at once, or try and write new data before it performs /PURGE on anything
• I have used /L attempting to get a clue, but it only lists what's marked for deletion, not when during the processes, the marked item is deleted