While Si is not memory, I believe Si is tied to memory;
Si is the subjective impression made after absorbing info via the fives senses, which can manifest as a memory.
Nostalgia is simply having happy personal associations with the past.
I think depending on your type and how you value Si (and since all types engage some Si), memories can manifest in different ways,
--Se valuing SEE may not think too much about past, preferring the present
--Ni valuing ILI may be critical of the past, deeming it non-meaningful
--Si PoLR ENFj may have a harder capacity to remember their past impressions
So how do you guys think about your memories and nostalgia?