In your tutorial

Is it possible to group and ungroup those rectangles? I mean, it should be possible that since you did it (I believe) but how did you? And is there a way to undo it?

it is possible (answering a question you directed towards Kirupa)

shift click on the objects you would like to group. Click on arrange and group. To undo it… click on Arrange (ungroup).

That should do it.

Good luck!