Location of iPhone and iPad backups

... I can see no 'General' tab in the Finder (as promised in the help screen)

Those instructions are a bit unclear.

First select your iPhone in a Finder Sidebar. You should see a window similar to the following, which unfortunately shows the General tab mostly obscured by the dropdown window that appears after clicking Manage Backups:

The Apple Support document really ought to be amended as follows:

To find a specific backup:

1. On a Mac with macOS Catalina 10.15 or later, open the Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes. Click your device in the Finder Sidebar. In iTunes, click your iPhone device icon.

If the Sidebar is not appearing in your Finder windows, please refer to this excerpt from Customize the Finder toolbars on Mac:

Hide or show the sidebar: Choose View > Hide Sidebar, or View > Show Sidebar, in the menu bar. (If Show Sidebar is dimmed, choose View > Show Toolbar.)

If you are not seeing your iPhone in a Finder Sidebar, we need to fix that!

Start here: Sync your iPhone, iPad, or iPod using your computer - Apple Support. Click the Sync with Finder option (obviously, but people with older Macs may need to Sync with iTunes instead).

Then, begin with Step 1:

1. Open a Finder window and connect your device to your computer using a USB cable.

That's probably the step at which you're getting stuck. If you need more help, please write back.

Assuming that works for you, continue with the remainder of those instructions: "After you set up syncing with the Finder using USB, you can set up the Finder to sync to your device with Wi-Fi instead of USB."

And then, everything works like magic.