Typically, with extroverts, things start externally, ping the extrovert, and the energy is directed back toward the external. This is also why extroverts often are described as thinking and feeling "on the outside" rather than the inside. The internal world exists to inform the outer.
With introverts, the process is reversed. Things are generated in the internal world, are shot out in the environment like radar, then ping back to the introvert, and the energy is used to impact the internal world. The focus is typically the internal world. The external world exists to inform the inner.
This is why extreme introverts are typically internalized (sort of like a tree that goes deep but has no foliage) and extreme extroverts seem to be all foliage but no core. Most people, though aren't extreme and have some degree of both.
Look at Se and Si as an example -- maybe the infamous chair example. Se sees the chair just as it is. Maybe that image of the chair impacts Ti/Fi, but this energy immediate pings back out into the environment and is focused on the chair. The actual image of the chair is the focus of the energy, and the person responds to it. This is also called an "objective" response because the object is external and thus accessible to all in the same form.
Meanwhile, Si typically already has an image of a chair inside. Perception is aimed outward, sees the chair, and data about the chair pings back inside, and the introvert is focused on comparing it to the internal image of the chair. The result might be to decide the external chair is not a good chair, or it could be to update the internal image of the chair to a better one, or something else; but the response is "subjective" because it all happens inside. The focus is to somehow impact the interior world of the introvert.
Extroverts tend to focus on improving/fixing the exterior world, the exterior world directs them, and energy returns to the exterior world; with introverts, the focus is the interior world, the interior world directs what energy they send out, and the energy is taken back into the interior world.