SimpleSSHD is a free (open source, no ads) SSH server based on dropbear that supports scp, sftp, and rsync. It only supports public-key based authentication (no password/interactive auth). It does not use root, which means it must listen on a port over 1024 (defaults to port 2222).
2020/01/01 Version 24: Fix crash with "Start on Open" on Android 9. New TV icon. Improve performance.