Null and Alternative Hypothesis

Hello, 

I have a question about the hypothesis statement.
In the video on "Confidence Intervals, two means, dependent samples" with the same magnesium example, it was mentioned that to construct the confidence interval, the "Difference" column would have to be: Difference= after - before, but in this same video example to construct the null hypothesis it is the opposite (Difference=before - after). 

On the other hand, in the additional exercise proposed for this video with the weight loss program, the column is changed again, since it is established that Difference= after - before.

This calculation is important because it is the basis of the data for the beginning of the statistical analysis and for the hypothesis statement, could you give me more details on how to get the difference and how to state the null hypothesis?