I am brand new to a Linux operating system.
I just loaded Debian 10 on a separate partition from Windows 10 (which I have only been using a few days).
The installation was very successful.

After logging into the KDE environment, a second monitor was detected.
Both the laptop top monitor and the second monitor were fully operational.

“It” asked me how I wanted to set up the monitors. I choose the highlighted icon, which I assumed was the current configuration. In fact, I was not really finished reading all the options and suddenly it switched to exclusive use of the second (more remote, older, poorer resolution and smaller) monitor.

My Question:
Now, I cannot find a way to manipulate the configuration of my two monitors. Minimally I want to choose the laptop monitor. Ideally I want the second monitor an extension of the first on the left side of the window.

I looked under display settings and can find nothing about dual monitors.

I hope I don’t have to write code to fix it or do write a bunch commands at the command line - wherever that is.

While Debian doesn't ship a very recent Plasma, I don't think the setup has changed much:

In System Settings -> Hardware -> Display and Monitor, you need to set the monitor as your primary screen where you want to have your panel. You can drag the other monitor to the side you want it to be, and check if the individual settings are correct (resolution, refresh rate). Apply the settings after making sure the specific setting is set to saved.

Usually there is not much else to do. I also suggest you set the session to be saved on logout, to make sure the windows stay where you want them:

System Settings -> Workspace -> Startup and Shutdown -> Desktop Session