Why use robocopy? It’s a good tool for a specific task but this is not the one.
You can simply use what cmd already gives you:
for /r %f in (*) do @copy "%f" target
This will essentially “flatten” your directory hierarchy. for /r will walk a directory tree recursively, looking for file names matching the given pattern. You can also specify the directory to start in:
for /r FolderA %f in (*) do @copy "%f" target
Within the loop it’s just a simply copy of the file into a specified folder.