Would you embed order items inside an order or store them in a separate collection? What factors would you consider?

Short answer

Embed them in almost every commerce model: they are read with the order, not shared, and bounded. Split them only if a single order can have an unbounded number of lines, or if lines are queried heavily on their own without the parent.

Why?

A receipt is an aggregate. Quantity, name, and unitPrice belong together so a later catalogue price change does not rewrite history. Independent item documents make sense for warehouse pick-lists at huge line counts, not for a typical checkout order.

Interview tip

Mention the price snapshot in the same breath. Embedding without snapshotting live catalogue price is a business bug.

Common mistake

Referencing products only by id on the line with no name or price, then $lookup for every historical order.