Help understanding collections / data implementation

Hey all, I am confused as to when exactly I should utilize collections and when not. For example, if I want to launch an app where users can upload and do catalogging for certain items, should i be creating a document for each item they upload or should the items be added to a list<Datatype(item)> under each user's respective document User?

Forgive me if I did not explain myself, but essentially I am trying to figure out for what scenarios I should create documents in collection and which scenario I should create lists of datatypes instead.

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