andrewteg
primitive ftpd + material files seem to be the most commonly suggested way to achieve this, on this forum.
On the main profile you can set up a local sftp server (make sure it is restricted only to connections from localhost).
On other profiles you can install MaterialFiles from where you can connect to SFTP shares and get access to the shared files.
One thing I am missing in this setup is lack of ability to mount such an FTP directory as if it were a local directory for a profile, so any app could use it normally, instead of having to necessarily go through MaterialFiles or apps like that.