db={orders:[{_id:ObjectId("65fee31937b6938d0107afe9"),products:[{_id:ObjectId("65fee0f137b6938d0107afe3"),qty:20},{_id:ObjectId("65fee0f137b6938d0107afe4"),qty:2}],uid:ObjectId("65fee0b337b6938d0107afe1")},{_id:ObjectId("65fee31937b6938d0107afea"),products:[{_id:ObjectId("65fee0f137b6938d0107afe2"),qty:4},{_id:ObjectId("65fee0f137b6938d0107afe4"),qty:1}],uid:ObjectId("65fee0b337b6938d0107afe0")}],users:[{_id:ObjectId("65fee0b337b6938d0107afe0"),name:"User 1",address:"Address 1"},{_id:ObjectId("65fee0b337b6938d0107afe1"),name:"User 2",address:"Address 2"}],products:[{_id:ObjectId("65fee0f137b6938d0107afe2"),name:"Product 1",price:30},{_id:ObjectId("65fee0f137b6938d0107afe3"),name:"Product 2",price:10},{_id:ObjectId("65fee0f137b6938d0107afe4"),name:"Product 3",price:40}]}

db.users.aggregate([{"$lookup":{"from":"orders","localField":"_id","foreignField":"uid","as":"orders"}},{"$lookup":{"from":"products","localField":"orders.products._id","foreignField":"_id","as":"productDetails"}},{"$addFields":{"orders":{"$map":{"input":"$orders","as":"order","in":{"$mergeObjects":["$$order",{"products":{"$map":{"input":"$$order.products","as":"product","in":{"$let":{"vars":{"productDetails":{"$arrayElemAt":[{"$filter":{"input":"$productDetails","as":"pd","cond":{"$eq":["$$pd._id","$$product._id"]}}},0]}},"in":{"$mergeObjects":["$$product",{"name":"$$productDetails.name","price":"$$productDetails.price","total":{"$multiply":["$$productDetails.price","$$product.qty"]}}]}}}}}}]}}},"productDetails":"$$REMOVE"}}])