Reinventing the wheel.
OpenSSH server (or ftpd if you're so inclined to reduce overhead) on the Linux machine and Total Commander on the Android device. Full control, put the files where you want them to go the first time.
Some of these are useful for copying files to friends' phones or other devices (though Total Commander has that covered too as long as its same network).
File/folder syncing is a different use case and deserves its own article.
¢¢