Can I combine several Picture in Picture windows into tabs?

Yes. Open the PIP windows you want to view, then right-click or Control-click a picture and choose Merge All Windows. BetterDisplay combines the viewing windows into a tabbed window and makes their title bars visible.

The action appears when more than one PIP window exists and is available when there are separate windows to merge. These are PIP windows, not Video Filter Windows. PIP requires Pro and the applicable screen-capture permission.

Use Show All Tabs in the picture’s context menu to see the tab overview. Choose Move Tab to New Window to separate the selected picture again.

Tabs organize the viewing windows. The source desktops and their apps stay separate. To interact with a source, use the usual PIP pointer actions. See Picture in Picture for tab and window settings.