The Video Preview window manages the video stream immediately received from the webcam or the primary tool for video capture. This is a “private” window that allows you to control your settings.
The function is activated/deactivated through the windows menu or with the general toolbar icon.

If there is no video capture tool, “no video” is written inside the window.
Clicking on the background inside the window with the right mouse button the audio and video configuration menu will appear, which functions similar to the horizontal text menu.

Preview window, particularly: video capturing configuration