Can I use a 2020 Mac Mini as a file server

Sure.

Just turn on File sharing on the Mini, and specify who has access to what files.

Set up file sharing on Mac - Apple Support

if you need a bit more speed, connect to your Home network with Ethernet.

For even faster access, add a network Switch with 10G Ethernet-over-copper wiring and connect using 2.5G, 5G or 10G Ethernet.

Lamposts wrote:

Thank you for you response it is now working, i can see both computers on each other, however the 4Tb drive attached to the Mac Mini is seen, and I have tried to relocate access to that drive into the 'Locations' section to no avail.

I could be wrong, but I don't think you can place the mini's external drive (the 4TB drive) under Finder Locations on the MBPro ... Locations will only list the server (the mini). I just tried it myself again to test it and was unable to put a remote drive under Locations, only the server it was attached to.

And it's not an option in Finder Preferences, either:

I was able to create an Alias to the remote external drive and put the Alias on my desktop and connect that way. But could not put the Alias under Locations in the sidebar.

Just adding a note to @Lamposts ...

After you enable File Sharing on your Mac mini, you will use the Finder on your MacBook Pro to connect to the mini. You would connect (aka sign in) using one of the mini's user accounts (account name & password).

In a Finder window (left panel) on your MBPro you should see the mini under Locations. If you only see Network, click it to disclose a list of the available servers. Either way, once you see the name of the mini, double-click it, then click Connect As and enter your username & password (again, a user that exists on the mini).

What kinds of items appear in the Finder sidebar is controlled by

Finder > settings > sidebar

this very old graphic is no longer completely accurate, will give you the idea: