Pay frequency is just division
Weekly, biweekly, semimonthly, and monthly options take the same annual estimate and split it. Biweekly is 26 checks, not two per calendar month. Semimonthly is 24. That is why two “twice a month” settings are not interchangeable.
Hourly mode converts hours and rate to an annual gross first, then follows the same tax path as a salary.