What to choose for null hypothesis and alternate hypothesis?

In this article which explains types of t-tests, for “Independent Two-Sample t-test”, null hypothesis was that the average screen size of the sample does not differ from 10 cm. and alternate hypothesis is that the average screen size differs. And also the data is only about sizes of screens.

Not just this example, in every article I have read, I always think that why can’t the statement chosen for alternate hypothesis be chosen for null hypothesis or vice versa. Also the data won’t have much information. It’s always upto the article writer to what to choose for null and alternate hypothesis.

So, my question is, are there some set of rules, that have to be followed to choose a statement for hypothesis. Or what actually causes in particular statement to be chosen for hypothesis. Or am I missing something.

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