Today I accidentally moved my mouse and probably clicked some button(s) on it and all the windows of my currently open apps appeared selectable on the desktop (I could select a window with the mouse). What kind of effect was that? How do I reproduce it? I know I can use Alt + Tab to switch between windows but that looked completely different.

The effect is called "Present Windows". Open Systemsettings, select "Desktop" and then select "Effects". Open the tab "All effects". Here you can search for it, enable it and click on the configuration button. Here you can set/search for the key shortcuts. If you select "Screen edges" instead of "Effects" you can select an edge to touch with the mouse in order to use this effect.
The names could be different, because I use German KDE and translated them.

Move the mouse to the upper left corner. Presents all windows all desktop. ctrl-f10
Move the mouse to the upper right corner. Presents all windows current desktop. ctrl-f9

You can configure this with system settings/desktop/screen edges.

Yes, that was it. Thanks.