21 is not an exact number of days, it is more for motivation than for determining the timing of the habit.
As well as the statement that 10k hours is enough for a person to become a professional in any field.
All people have different ways, it all depends on the discipline.
According to my observations, it takes 10 periodic repetitions to form a habit. Those. for a weekly it will take 10 weeks. But if someone thinks 21 is correct, then 21 is week.