If you need to rotate video a small amount to correct a camera tilt or create a special effect, you can't do it with iMovie or QuickTime Player. However, you can use Keynote to rotate video. This involves a small Keynote project with just the video, plus a rotation, and then an export. You need to make adjustments to avoid leaving blank corners.
If you have video recorded in the wrong orientation, you can rotate it after you add it to the iMovie timeline. You can also change the original file by opening it in QuickTime Player, rotating it, and saving a new version. If you need to rotate video in Photos, you'll have to export the video, rotate it in QuickTime Player, and then import the new version back into Photos.
You can export a still frame image from a video many different ways with the software that comes with your Mac. If you are already in iMovie, you can use the sharing function to export an image from the current frame in the timeline or a library clip. In Photos you can save a frame in a video to your Photos library as a picture, and you can also copy the current frame and paste it into another app. The easiest way is with QuickTime Player where you can copy the current frame and then paste it into Preview or any other app that handles images.
If you ever come across video files that won't open in QuickTime Player, you can use the free VLC Player app to view them. This popular app has been around for a long time and can also be used to convert videos to a more standard format.
You can use QuickTime Player for very simple video editing without setting up a whole project in iMovie or Final Cut Pro. With QuickTime Player, you can combine trimming, appending more clips, and splitting clips to arrange and export edited movies.
You can record your Mac's screen using only the QuickTime Player app that comes with OS X. It is easy to record and then you get to preview the result before saving.