Ni is the dominant function of both INFJs and INTJs, and it’s a perceiving function. As such, it just sits there and churns out alternatives and possibilities and back-up plans. No decisions are made.
But at some point that churning mess has to come out into the real world. So the churning mess comes out into the world through the auxiliary judging function (Fe for INFJs and Te for INTJs). The auxiliary judging function makes a call on what the final product should look like based on it’s own Te or Fe rules. So for INFJs, the results of Ni churning are whittled down and packaged up in an Fe model; for INTJs, the results of Ni churning are whittled down and packaged up in an Te model.
At this point, Ni may continue to churn on the problem further, and the Ni-dom may insist that they haven’t provided a final product yet. But if a product made it into the outside world at all, then it had to go through a Judging function, which means it was packaged at some point into a model for real-world application.
But since the Judging function (Te or Fe) is only the Auxiliary function in Ni-Doms, that means that the Te or Fe model-building function is pretty weak. To put it another way, a Judging “patina” of model-building was applied.
Similarly, the Fi models of INFPs get built from the churning of our relatively weak Ne (perceiving) Auxiliary function. Our Auxiliary Ne churns a bit (much more slowly then Ni in an Ni-dom), runs a few possibilities past us, and at some point our strong Fi-dom grabs one of those possibilities and hammers it into a strong Fi model.