How to choose Null or Alternate hypothesis for Z test

For hypothesis testing, I got to know how to do testing using z test and t test… but want to know how to choose the hypothesis…because choosing a statement as null or alternate hypothesis is not affecting the procedure or p value after calculating from formula.
If we interchange the null and alternate hypothesis statements…how is it afecting our result…could not figure it out…
pls if somebody could clear this doubt

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