As an attempt to understand the collaboration of functions, I've read several theories and made up my own theories to distinguish the difference between S and N functions. I'm gonna start with analyzing Si (Te, Fe) vs Ni (Te, Fe) functions.
We all know that in MBTI, Si dom users divided into two types, ISTJ (Si Te Fi Ne) and ISFJ (Si Fe Ti Ne).
While Ni dom users divided into two types, INTJ (Ni Te Fi Se) and INFJ (Ni Fe Ti Se).
I personally don't believe the theories that said Si recollected past memories and live based on their memories of the past. I think both Si and Ni works like an instinc. But since Sensory functions (Si and Se) prefer concrete ideas, they will rely more on realistic situations. This is of course different with iNtuitive functions who prefers abstract ideas and are more flexible.
Let say that there are four kindergarten students. They are ISTJ, ISFJ, INTJ, and INFJ.
One day, their teacher teach them how to coloring a gold fish picture. She show them boxes of crayons. Each box contain crayons with the same color. "Use the yellow crayon, children," said her.
The four students do what she said. They each take a yellow crayon from the crayon box and use it to coloring the gold fish.
Three months later, the teacher ask them to do the same thing. She give them a coloring paper with a gold fish picture. "Color your gold fish with the same color like you did three months ago."
Let say that the four of them already forgotten what color they use three months ago.
The ISTJ and ISFJ will use their Si in the same way. They look at the crayon box.
Let say the ISTJ take the red crayon and examine it. But then his Si, works like an instinct, suddenly say: "No. That's not the right color."
The ISTJ put the red crayon back. He then take the yellow crayon and examine it. And his instinct say: "Yes. This is it." So he keep the crayon and use it to color the gold fish.
Even though he didn't remember what color exactly was the crayon which he used three months ago, his Si made his subconscious mind to copy, whether it's the feeling, or the aura, or the sense that he got when he was using the yellow crayon to coloring the gold fish.
In the present, when he's choosing the right crayon, his Si bring back the sense and convert it into some kind of instinctual form. If the teacher ask him, does he remember the color of the crayon, the ISTJ will answer: "No, I forgot."
Teacher : "So how did you know that it's yellow?"
ISTJ : "I just knew."
How about the INTJ and INFJ? Since Ni doesn't work like Si, the Ni will rely more to the second function to memorize the crayon's color. To INTJ, this means he will use his Te. To INFJ, this means he will use his Fe.
The INTJ, using his factual Te, remember that the crayon's box was the third one from the left.
While the INFJ, using his impressionist Fe, remember the impression he got when he saw the color. He remember that the color made him felt cheerful and warm. As the result, the INFJ will probably take the crayon with the same impression. It is also possible that he will take an orange or red crayon, because to him, both colors have the same impression.
So when choosing the right color, the INTJ will be more precise than the INFJ. While the INFJ will be more creative than the INTJ.
A week later, the teacher ask them to draw a futuristic modern city. This is new for the students, as the teacher haven't teach them about it yet. The teacher expect them to use their imagination.
As Ni users, the INTJ and INFJ will draw a city with some things that they have not seen before. Like the flying trees, pink fishes riding a car, or futuristic buildings, etc. Their Ni works like an instinct that can predict things outside the realistic ideas.
What about the Si users?
Lack of abstrack ideas, they will use their Si to develop the future based on reality. They will draw the incredible highway, based from the real highway they have seen IRL. The concept of the city itself will looks similar with the real city they currently live in or the city they've seen before.
The ISTJ will rely on his factual Te to make sure that the street, the buildings, and the people will have the proportional and realistic size/composition. While the ISFJ will rely on his impressionist Fe to make sure that every things have the realistic colors, or make sure that the people were doing the realistic activities (shopping, working, eating, etc).
So basically, I believe that both Si and Ni works like an internal instinct. Their priority is to trust these instinct, whether or not the outside world agree with them. This internal instinct is balanced by the extroverted second function, whether it's Te or Fe, so the Si/Ni user will be able to consider inputs or stimulations from the outside world to strengthening their instinctual power.