Well NiFi are everyones functions in socionics(and in some socionics like type theories like beebes archetype theory), but not in MBTI or in jungian typology. In jungs theory NTSF are the functions of everyone and different types come from I/E of these functions and preferences to habitually use one or two of these functions over other functions.
Functions exist in the psyche even before they are developed to the point that their contents reach consciousness. If we are talking about averages of function development, its usually more like aux in teens, tert in early adulthood and inferior somewhere between 35-50's. But the thing is that not everyone ever develops much of the other functions than just dom and some might start to develop their inferior even in their 20's.
The theory of function order comes from the idea that judging functions represent opposite things in the psyche, and perceiving functions do the same. Like if you are logically analyzing something, it guides you away from analyzing it from personal point of view. Now if a kid learns that its best to analyze via logic instead of using personal point of view like how something makes me feel, or whats the atmosphere you feel in a situation, he will push all the F stuff in the unconscious and make a habit out of T. Or if someone is focusing on concrete perceptions about the external world, it takes the focus out of what might lie behind what is seen and if a person learns that its best not to focus on "imaginary" stuff, thats what he will do and push the imaginary stuff in the unconscious and make a habit out of S. Now we have a person whose dom-aux axis is thinking and sensing, but you can approach both thinking and sensing from different points of view, by the subjective factors or objective ones. I and E are opposites in the same way as T and F(or S and N) are. If you focus on the external world and adjust yourself according to it, its away from adjusting yourself according to subjective factors, however if you focus on the subjective factor when using thinking for example, you can focus on the objective aspects when using sensing to give you that more healthy balance. This is also why the inferior gets the opposite I/E from dom and tert opposite from aux.
But ofc some people, like ISFP for example, might be stuck in his Ni and use it more than his Se at some points of his life, but that doesent change type, it just tells something about the mental functioning of the person, which in these cases is usually unhealthy.