As far as I understand it, the Si/Ni difference is like... Si is the process of identifying static entities based on inherent qualities, in particular those that the person has experienced. Si rests on a Ne foundation, so there's linking of attributes. I remember reading someone's post where it was like if Si was presented the idea of "red dress", it would evoke all the red dresses the user had ever worn, seen, or otherwise experienced, creating a set of qualities that create "red dress". What "red dress" means to that individual would be an amalgamation of all the "red dress" experiences, direct or indirect, that they have ever had.
On the other hand, Ni is more like an underlying web of universal concepts, and the Ni user will seek those universals within the process of their Se experiences. So if you presented Ni with "red dress", instead of evoking experiences of red dresses, it would seek what the concept of "red dress" points to - special occasions... the "sexy" or "evil" woman wearing a red dress opposed to a white one... it looks for the collective underlying qualities. What "red dress" means to that individual is less specific and more cross-contextual because it's not about the red dresses themselves but about what "red dress" indicates about the situation.
So in terms of the question I was asking above, it's really interesting to talk to a Si-dom versus a Ni-dom about places, because Si-doms will tend to be very specific about places, to use their proper names and individual identifiers, and to sort of latch onto keywords describing particular entities that might not necessarily be universal, but have been prevalent in the Si-dom's experience. My ISFJ is from a larger city than my hometown, and I remember once he was talking about going to a "civic center" for something, seeming to assume we had them and that they were the same in functionality as in his hometown. Here, though, they're not as common, and they aren't as popularly used, so they're really not as much of a community cornerstone. We've had a few situations like this and he often seems pretty surprised when something is very different to his previous experience of that concept. A strong Si user tends to come to the world with much more rich, detailed, and experiential knowledge of what they are familiar with, which is a huge boon when they are in their element, but they might struggle a bit when things are very different from anything they've experienced before.
Whereas you talk to a Ni-dom about places, and they may very well not remember any of those specifics like proper name, exact location, etc., but they tend to "map" in terms of underlying purpose, and especially with INFJs, for meaning. So an INFJ will often really not be able to go into such rich and accurate illustration as an ISFJ about places they know, but they seem to do better cross-contextualizing, because they will tend to classify more by function, and as such they can identify a "community center" in a civic center in one town or in a public library in another town, as they're not looking at the specific structure itself but at its underlying universals that make the "community center" a concept prevalent all over the world, even though it is manifested differently in different places.
I have a coworker who I really struggled to type ISFJ versus INFJ, because she's a strongly phobic 6w7, so it makes her very classically SFJ-ish in terms of being alert and protective, but her communication is much more N. She struggles to voice particular detail, and tends to refer to things by what we use them for or what they do. That aspect, plus her strong belief that she is good at predicting the future (she is), and a handful of other details, made it clear that she was more of an INFJ than an ISFJ.
To be fair, I have more experience with Si doms and aux than with Ni dom/aux, and I'm not sure how true to Ni/Si perception I have been able to voice all of this - I hope it's not too Ne-tinted, but it may be.