How to extend vertical video with the same duplicate size on its side?
A great free online editing software is WeVideo. In WeVideo, any video that doesn't take up the whole screen (like video shot in portrait mode) can be layered on top of any other video by putting the video you want on top by putting that video on the video track above the video you want below it. The way to get your video to look that way you want it to would be to have the blue square with the words on in Video track 2, and the black background and lightning bolt on the lower track. Then, you could double click on the blue box clip, and it should bring you to the transform menu  where you can drag the clip that you have selected around until it is where you want it in the little preview screen. This way you can have the clip in the second track be anywhere in the screen. Hope this was helpful!