This entry is merely here so I don’t have to try and remember where I wrote it down for the next time I need it.
Problem A directory, A, contains many sub-directories, B, C, D, …. Each sub-directory contains one or more files that you want copied to a new directory without the directory hierarchy. Result will be a directory, A1, that contains just files.
Solution The *nix cp command.
cp -r <source_directory>/*/*.filetype .
Copy, recursively for files (little r, not Directories or big R), from the source_directory, searching all folders (/), for all files of a type (/.filetype) to here (.)