What constitutes a proper warm-up for football players?

This is more of a random post, however based on some discussion/comments I have received over social media after posting some warm-up of FC Bayern Munich, Real Madrid, Zenit FC, Schalke, Manchster United, the Spanish National team and elite youth teams of different nationalities.

The statements were about the content of the warm-ups of the teams. Furthermore, I would like to give some thoughts about different issues that might influence the approach to develop/do a warm-up.