Copy a Directory Tree "Flat" - all files into one folder, no subfolders

Let's say you have exported all families in a Revit project to a bunch of folders by Category, but you want to get them all into one folder so you can drag-drop and load them all into a new project all at once...

Copy the following into a batch file:

@echo off
set source="E:\tempE\export"
set destination="E:\tempE\export2"

::Not sure if this is needed
::It guarantees you have a canonical path (standard form)
for %%F in (%destination%) do set destination="%%~fF"

for /r %source% %%F in (.) do if "%%~fF" neq %destination% ROBOCOPY "%%F" %destination% *.rfa /COPYALL /R:0

Now, change the text in red to suit your source and destination (my destination folder was empty).  Run the batch file and you should have a stack of RFAs all sitting in that folder.  Note: results may be a bit unpredictable if you have duplicates of particular files in the source directory tree.

Read more:
file - ROBOCOPY - Copy folders content to a single folder - Stack Overflow