Plasma Pager widget shows by default only greyed previews (or outlines) of opened windows in virtual desktops. Previews reflect windows position and size on a desktop. No app icons are shown on previews by default.
To enable app icons on greyed previews:
- right-click on a pager
- select Configure Pager...
- enable "Show application icons on window outlines" option in General section
Though app icons are very hard to read imo, as on the screenshot below, which has:
- dolphin at top left
- kcalc at top right
- kinfo at bottom right
- pager at bottom left
Three apps are opened on the 1st virtual desktop. It's in the top left quarter of the big Pager widget at bottom left. Three other desktop don't have opened apps.
Pager settings aren't applied globally to all Pager widgets. If there are 2+ Pagers on a desktop, each one is configured independently from another.