With the project loaded, expand the timeline for best view by dragging the slider knob at the bottom-left of the window to the right.
Remember, video on higher-numbered tracks on the sequence takes precedence over video on lower-numbered tracks.
Here, a transparent Apple logo has been placed on track 2, and the result...

If a background (lower-numbered track) video has no watermark, then it's being added on a higher track. Remove that overlay track, or click the HIDE TRACK ON OUTPUT icon, the "eye" at the left side of a track.
If the background clip on a lower-numbered track HAS a watermark, then it's embedded into that clip and cannot be removed. Replace it with the original video that had no logo/watermark.