Can you write comments for this code: db.order.aggregate([
  { $match: { customer_id: "642d33d2c59828083ad22c18", _id: ObjectId("642dbb317d02c359ef57d44a")
 } }, // Replace CUSTOMER_ID_HERE with the actual customer ID
  { $unwind: "$books" },
  {
    $lookup: {
      from: "book",
      localField: "books._id",
      foreignField: "_id",
      as: "book"
    }
  },
  { $unwind: "$book" },
  {
    $project: {
      title: "$book.title",
      author: { $concat: [{ $arrayElemAt: 
["$book.author.first_name", 0] }, " ",{ $arrayElemAt: ["$book.author.last_name", 0] }] }, // arrayElemAt gets first element of array since only one author
      cost: "$book.price",
      _id: 0
    }
  },
  {
    $group: {
      _id: null,
      books: { $push: "$$ROOT" },
      total_cost: { $sum: "$cost" }
    }
  },
  {
    $project: {
      _id: 0,
      books: 1,
      total_cost: 1
    }
  }
])