files share with Mac (Catalina) show up with <no access> permissions.

Should the client version be a file server? Maybe it will as they are cutting those features out of the Server app, but for now, you have to configure the file server functions manually. File sharing is just making the directories available to specific users. You can share files, but the implication is the other users will make a copy of their own.

There are some apps to put those ACL creation strings behind a GUI, but I prefer to know what it is doing if I'm managing a file server.

By default, files are create with the user who created it as the owner, the group of the folder that it is in, and others (everyone), no access. Everyone > No access doesn't mean all users are denied access. It is actually the POSIX "others" permission. Others makes perfect sense, everyone makes no sense.

If you want to have multiple users edit files created by others (scanner, sharing users), you need to set up an access control list (ACL) to allow that. File Sharing does not equal File Server.

You need to create a group in Users & Groups (same as creating a user, just change to group), and add all your users into that group. You'll need to figure out what user you scanner is using (maybe nobody which might be difficult).

Here is the ACL and command to add to the folder, then apply to enclosed.

chmod -R +a "group:<groupName> allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" "Path/to/directory/"

Replace <groupName> with your group name and "Path/to/directory/" with your desired directory, /Users/Shared/

I would create a folder inside of Shared to apply this to, but I don't think I have any good reason.

Once you edit the parts in a text editor, copy and paste it into Terminal and hit return.

The, Get Info on the folder and select Apply to Enclosed from the gear menu. If you don't already have subfolders, you don't need to apply to enclosed. Any new folders will get the ACL inherited.