Add a second video that will be spliced into your main video. Drag the amber block on Video Track 2 to reposition it.

Overlay floats an image on top of the video for a window. Insert splices the image into the timeline and adds its duration to the output. Drag the green block on the Graphics Track to reposition it.

Mix background music under your video. Source video volume applies to the original audio; each music track has its own volume, loop, fades and time window.