What exactly is the difference between the answers to Why? and How?

Why? The answer(s) to this question describe the reasons for behavior. The reasons why a certain state of reality is wanted/needed. It’s the description of the reason(s) for a goal that is wanted/needed to be achieved. It’s the main driving force behind the behavior.

How? The answer(s) to this question describe the sequence of events leading to a resulting goal. The road to achieving a certain goal can be subdivded in many smaller steps/goals. The main goal is the answer to the Why? question. The different sub-goals are part of the How? answer. The Ready Fire AIM process defines the sub-goals while traveling on the road towards the main goal : the answer to the Why? question.