Examine this aggregation. What does each output field of the single result document contain?

db.products.aggregate([ { $match: { active: true } }, { $facet: { byCategory: [ { $group: { _id: "$category", n: { $sum: 1 } } } ], priceStats: [ { $group: { _id: null, min: { $min: "$price" }, max: { $max: "$price" } } } ], top3: [ { $sort: { price: -1 } }, { $limit: 3 } ] } } ])