What to choose for null hypothesis and alternate hypothesis?
You always choose the statement that you want to disprove as Null hypothesis. You can either reject it in favour of alternative hypothesis or not reject it. We don't say we accept null hypothesis. Because we don't have enough evidence for the null hypothesis to be true. It might be true, might be not. But with significant t/p value we can reject null hypothesis in favour of alternative hypothesis
Like in this example, if you don't get any significant value after the experiment you do not reject that the average screen size of the sample does not differ from 10 cm.But you don't have any proof to accept it either.
Always choose the statement you want to disprove as null hypothesis.