Si is a database or hard-drive and Ni is a logical processing unit or a CPU. The first provides statistical pattern recognition based on classification. Si is concerned with comparing and contrasting. Ni is concerned with reducing the information down to a single universal principle and it wants to conceptualize which is necessary to make predictions.
Si is analogous to experience and Ni is analogous to wisdom. The first suggests having found out what works and the second suggests guiding principles to help navigate through new and challenging experiences. It is possible to have wisdom without experience, but without experience one might not fare so well through all the treacherous detail one is likely to face in life.
Ni informs that a tomato is a fruit, Si informs not to put it in a fruit salad.
Si answers “what is it” and Ni answers “why is it like that”.
When my Si-using wife (ISTJ) tastes something she can relate it to something else she’s had at a particular restaurant, and she may make adjustments while cooking to get to a result that matches her memory. When she asks me to taste it, I think of the different regions of the tongue (sweet, sour, salt, bitter) and I determine what the dominant note is, then I determine if it is balanced in those regions, if not then balance sweetness with something acidic (vinegar, wine, vermouth, etc…). So my Ni conceptualizes and narrows down the options to a handful of things that would theoretically work. We have a different approach.