Combine songs to one video

You can use PIP (Picture in picture) feature in Premiere elements to to this..

First add a video on Timeline. Now drop another video from Project Assets (In expert room) on the Monitor.

when you drop you will get option to create PIP 

Another way is to use Video Collage feature. 

There are templates where you can add upto 7 videos in a frame (Create > Video Collage)

It is going to take some serious work and patience.   Basically you put the piano track on the bottom.  Then you put the videos on tracks above, size them and position them.   It may take a lot of whisky to get through it.  Just asking the question suggests you are unfamiliar with video editing.  That is a very advanced project. 

That said, Premiere Elements does not have any significant multi-camera tools.  So it has to all be done manually, tediously and carefully.  It may also be that that video was produced with one of the "meeting" apps.  

This is possible but is extremely difficult to do in Premiere Elements (or pretty much any program). The video you're trying to recreate used no less than 16 video tracks and, if recorded remotely, mixing at least 16 different audio tracks AND keeping the whole thing in sync! It's not for the novice.

As I said, it's possible -- but not without a lot of work and a very high level of understanding and experience.