Recording video (with audio) while playing original vid on Quicktime

I just got a bunch of old VHS tapes digitized through the Legacybox service.

Downloaded one of the files.

I want to capture an individual portion of the video.

When I open the video file, it opens with Quick time.

Then I go to "new video recording" and it seems to capture only a jpg image.

Which makes me ask, can I run a source video on Quicktime and at the same time do a screen recording?

Is there another way of cutting up pieces of video on the 1 gig file ?

thanks

Greg