Maybe it is not best explained, but the theory is accurate, another link: https://americanpregnancy.org/healthy-pregnancy/week-by-week/pregnancy-week-1-2/

how the baby’s body is going to chage fast in the first week?

You bet, it might be going from not existing to existing, that is some change.

It is really awkward, but it is due to a practicality, it is very easy to pinpoint last menstruation, but very hard to know when conception happened.