Peer to Peer networking cannot be done if the machines are on a domain.

https://www.windowscentral.com/how-setup-and-manage-windows-10-homegroup-local-network

Set up a HomeGroup to share libraries and devices with other computers on the wired network. You can also stream media between devices.

HomeGroup hasn't changed much from previous versions, as such you will be using Control Panel to make the configuration.

1. Open the Start menu, do a search for HomeGroup and press Enter.

2. Click Create a homegroup.

In Windows, search for and open Homegroup.
If a homegroup has not been created, click Create a homegroup.
In the Create a Homegroup window, click Next.
In the Share with other homegroup members window, select Shared in the box next to the folders or devices you want to share, then click Next.
Step 3: Sharing drives, folders, and files

To access the computers on your wired network, you must turn on network discovery. You can also set files and printers to be shared as well as set sharing options for specific files or folders.

Turning on network discovery and file and printer sharing:

Follow these steps to open advanced sharing settings and turn on network discovery and file and printer sharing on your wired network.

In Windows, search for and open Network and Sharing Center.
Click Change advanced sharing settings.
Select both Turn on network discovery and Turn on file and printer sharing.
Click Save changes.
Setting sharing options and permissions for specific files or folders:

Set sharing options of files and non-public folders from the computer whose content you want to share. To share non-public folders on your wired network, do the following:

In Windows, search for and open File Explorer.
Browse to the folder you want to share.
Right-click the folder, select Share with, and then click Homegroup (view), Homegroup (view and edit), or Specific people.
If you choose Specific people, the File Sharing window is displayed.
Click the down arrow and select the user you want to share with. If the user is not listed, type the user name and then click Add.
Click an arrow under Permission Level to set the permission level for each user or group.
Click Share.
Step 4: Testing the wired network:

Check the network by browsing through the shared folders on each computer on the network. To verify the setup of your wired network and browse the shared folders, follow these steps:

In Windows, search for and open Network.
Double-click the name of the computer or device to access.
If prompted, enter the user name and password to connect to the computer or device.If the computer is able to read and access files from a remote computer, the remote computer or device is set up correctly. Browse to every available computer or device from each computer on the network. If there are any issues, go back through these steps and verify that the settings are correct.If you are unsure how to browse shared folders or are experiencing difficulties, refer to the section Accessing shared files and directories.When all computers are able to network to each other computer on the network, continue to the next step.
Step 5: Enabling Internet access and firewall:

Once you have verified that your home network is capable of transferring files, connect and enable Internet connections for computers with Internet access.

Note: Make sure that each computer with Internet access is well protected from security threats. At the minimum, each computer should have its Internet connection protected with a firewall and Windows should be updated with the latest critical updates from Microsoft Windows Update. If malicious activity comes though one computer, the activity can quickly spread through the entire network.

Thanks for trying, Speedstep, but Homegroup was removed from all Windows 10 versions beyond 1803 (which is why I mentioned my v.1809). I guess that's part of the problem. What they call an "improvement". Any other ideas, anyone?

Forgot about that.  1809 is almost done 1903 is a few days away.

Pretty sure they did the removal of HomeGroup so that you have to use the cloud (Get a link you can share with anyone. Right-click (or press and hold) a OneDrive file or folder and select Share a OneDrive link. ) or one of their APPS.

With the greatest respect, speedstep, why should I want all my work passed through the grubby hands of Microsoft to some server in the sky when I simply want to print a document or share a file in the next room of my own home?

The problem is only in one direction. I have installed a printer driver on PC-B and moved the printer to a USB port on that machine. It is completely transparent to PC-A, as it should be. The problem arises when doing it the other way round - trying to access the printer on PC-A (Dell) from PC-B. Yet both computers appear to be configured identically!

Surely it's more than a coincidence that we have difficulty accessing PC-A directly from the file explorer on PC-B, unlike the reverse. Help!