I have 2 lists holding the same data type that has unique “id” property. One is a bigger list that serves as a single source of truth and the other is a sublist. After the big list gets updated, how can I go through the smaller list and update values present there from the bigger list when “id” matches?

I would transform the bigger list into a Map<id, Data> (using associate), then I would loop over the small list, for each element, find the matching one in the bigger list, and update.