I understood this notsomuch as "how exactly did they do everything in this video" and more as "I wish to check and reject unfilled bottles somewhat like this".
We really need to know more about the project.
But the clues this far, like the level of knowledge, hints to keeping it simple.
That's why I go with a more primitive industry-automation type solution instead of steppers, loads of sensors and tons of code.
The ideal solution to keep it simple would be to combine the detection and rejection into one position.