this two-part post, Jung's function stack called for the auxiliary to have the same attitude as the dominant.
And finally, as further discussed in this post, Jung attributed the ambiversion of the "middle group" to the fact that they were "less differentiated" than his types.
Jung viewed extraversion/introversion as the most fundamental division underlying his types, and spent more of Psychological Types talking about the personality characteristics he thought extraverts tended to have in common and introverts tended to have in common than he spent talking about all eight of the functions put together. And Chapter X — the only part of Psychological Types where he gets into the functions in any detail — is organized accordingly. The first half of the chapter is devoted to "The Extraverted Type" and the second half to "The Introverted Type" — and the eight "function-types" consist of four varieties of the "extraverted type" and four varieties of the "introverted type."
By contrast, the ambiverted "middle group" that Jung said was the most numerous of the three groups (and represented the "normal man") was made up of people who Jung said were neither extraverts nor introverts.