So, I'm using Plasma 5.23.2, and I click the "show desktop" icon and all the windows go away and I see the desktop. So far so good. Then I look through to task bar to see which program I want to see. I click on its icon, but the ALL the apps come up. This is not a desirable behavior. Why would I want to "show desktop"?

Yea, my wallpaper is neat, but I only want one window open.

How do I do this?

The "Show Desktop" function toggles between hiding the entire desktop or showing the entire desktop. I don't think it has the functionality you describe, that is, to hide the entire desktop, then show only one application when several are open. When you "unhide" the desktop, you "unhide" the entire desktop.

Frankly, it's a function I've never needed, but I can see how persons doing something sensitive might want to hide the desktop from view in certain situations, particularly if they are working on something sensitive.

hmm.... Now I'm thinking the thought process was that so people could open icons that are located on the desktop. (I use the mode where there are no icons there) When I just have a dozen programs open and it's too cluttered.

I don't use Plasma, but if I were you I'd try to make sense of its keyboard shortcuts. E.g. Ctrl+F12 followed by Ctrl+F7.

Ctrl-F7 doesn't do what Hedron wants.

Ctrl-F12 followed by Ctrl-F7 seems to behave identically to pressing Ctrl-F12 twice.
If the current application has one window, Ctrl-F7 doesn't do anything at all.
If the current application has multiple windows, it hides everything but those windows, allows you to select one, then returns to as before with that window selected.

As far as I can see, there's no "minimize all" option in Plasma, nor is there a "hide all except current" option - both of which would do what is desired.

However, it should be possible to write a script with xdotool to do this (then bind it to an appropriate key).