Without any file manager, subdomains (sub-servers) are totally useless and unusable.
How can I get a file manager on a subdomain?
I don’t understand what you’re asking here.
A sub-server is owned by the parent Virtual Server account. If you enable File Manager for the parent user, then the parent user can browse all of their sub-servers (which live in /home/parent-domain-name/domains/ by default). So…sub-servers do have File Manager.