Thanks for liking the article :)
Regarding the fullscreen button, you want to create a layout file called exo_playback_control_view. ExoPlayer automatically looks for a file of this name if you want to change the layout of the media player. You’ll need to define all the buttons yourself at that point, but you’ll then be able to add a fullscreen button.
A good article going over how to do this can be found here: https://medium.com/google-exoplayer/customizing-exoplayers-ui-components-728cf55ee07a