It depends on the cantons...
Annual taxation : GE VD VS FR TI
The program calculates each month the taxes since the beginning of the year (or the date of entry of the employee) until today. If it has been asked to take into account a future 13th salary, it will take it into account (see settings here). The fact of annualizing or not the salary elements is defined in the parameterization of the salary items (a punctual element qualified as a-periodic in the headings will not be annualized).
Monthly taxation : all the other cantons
The program calculates the month within the month without looking at what happens before and after.
More generally, the program calculates taxes according to the new federal law that came into effect on January 1, 2021.
Below, you can download the text in French, German or Italian.