So our preference for F function might be related to our interaction with parents as infants. Our physiology at birth might also be predisposed towards N or S and F or T. So nature and nurture.
So perhaps we are born as our core type like NF NT SF ST and our interaction with the parents and the world afterwards affect the order and the direction of the first two functions. In my case, I can say that my tertiary become noticably strong enough around my late 20s. So until that time, I was predominantly running on my first two functions I guess.
We adapt to secure nurturing from our parents. Fear of abandonment and death.
I was ambiverted as a kid yet gradually become introverted in my teens. So external factors and pressures of life forces and molds us into our strongest combination of first two functions and we end up having to develop our tertiary and inferior functions in relation to that.
Assuming we were running on dominant and auxiliary functions till late 20s, one of the functions must have served to interact with the world to gather information and affect the environment and the other must have served for internal processing purposes to analyze the internalized raw data. I guess the perceiving functions are related to id or superego and are autonomous whereas judging functions are related to ego. So as infants, our perceiving function in either the dominant or auxiliary position must have served as an autopilot for us to learn from the environment and develop our egos. The perceiving functions must be containing some kind of genetic inherited knowledge of species, some sort of source code that enables offspring of species to survive in the environment. Like kittens already knowing certain things by birth or instinct.
So let's say we are born as an pseudo-NF and our interaction with the external world puts our F to the dominant position and our N to the auxiliary position. Then we become pressured by the realities of external world and have to adapt a direction for the dominant and the reverse for the auxiliary so that one of the two active functions serve as our connection to the external world.
F is preference for spiritual (romanticism) whereas T is preference for material (realism). So someone who gives more preference for F will have proportionately less preference for T.
Likewise S is preference for facts\reality whereas N is preference for surrealism\mystical. So someone who gives more preference for S will have proportionaley less preference for N.
So our core type settles till late 20s and by then, we also have indirectly settled our tertiary and inferior, which we began developing from that point on, now that the first two functions have reached their limit for growth.
I don't know what happens when both of the first two functions are introverted or extraverted by birth. The first case represents someone who has no connection to the outside world whereas the second one represents someone who keeps on internalizing information from the outside work but has no internal analysis mechanism to analyse the data.
Anyway, just wild assumptions like I said.