The formulas are public: each page shows its calculation and assumptions below the result. The default values (rate, term, return) are editable examples, not Hyppo's or a lender's terms.
Each simulator also states what it does not take into account: insurance, guarantee and arrangement fees, taxation, depending on the tool. The calculation shown on the page is redone identically on the server when you ask for the result to be sent as a PDF, and it is checked by automated tests on reference cases.
For a fixed-rate property loan, the loan offer includes an amortisation schedule (article L313-25 of the French Consumer Code): the simulator gives an indicative version before you take any steps.