I'm running Ubuntu 20 and have installed Pure-FTPd. When I SFTP on port 22 with FileZilla the log shows "We claim SSH-2.0 FileZilla ... Remote version SSH-2.0 OpenSSH..." On login with FileZilla, auth.log shows an ssh2 connection.
I'm confused about why I don't see something like "Welcome to Pure-FTPd [privsep] [TLS]". The service is running. I haven't read anywhere that we need to do anything to tell OpenSSH to hand over SFTP requests to another process. I'm guessing that something else needs to be set under /etc/ssh/sshd_config.d ?
It doesn't look like the PureFTPd service is processing these requests at all, with its own user db or other settings.