Without trying to give a dictionary definition of the phrase (which is impossible) let me say the the OP quintessential example of a movie formula is The Man With No Name trilogy.
In the modern era, we've all heard the nonsense about the Marvel formula. Iron Man trilogy had a formula. Captain America trilogy had a formula, but it was far different from the Iron Man formula. Thor had no formula at all, it floundered between one style after another until it found something that sold well. Ant-Man had a formula, Guardians of the Galaxy has a formula, but there's no mistaking that from other Marvel formulae (other than the last Thor film), Mos, BvS, and ZSJL have the same formula, but WW had a different formula and you can't mistake the two. Aquamarine and SHAZAM had entirely different formula again, though they haven't had sequels (which can also be said of the original JL)
What else has a formula, and how is it distinct from other formulae?