Challenge 3: Design a $lookup Join — Possible Solution
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{ $lookup: {
    from: "products",
    localField: "product_id",
    foreignField: "_id",
    as: "product_details"
} }

WHY THIS WORKS AS AN ANSWER
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from: "products" names the OTHER collection being joined in — the one
holding the data we want to attach, just like the table named after
JOIN in SQL.

localField: "product_id" is the field on the CURRENT collection
(orders) holding the value to match on — orders.product_id.

foreignField: "_id" is the field on the OTHER collection (products)
that value gets matched against — here, the product's own _id, since
product_id is presumed to store a product's ObjectId.

as: "product_details" names the new array field attached to each
output order document, holding every matching product document (in
practice, exactly one, since _id is unique).

Worth noting: $lookup always attaches an ARRAY, even when — as here —
there's exactly one match. A follow-on $project or $unwind stage is
usually added afterward to pull the single matched product out of that
one-element array into a plain object, which Chapter 2 covers directly.