db={"products":[{"_id":ObjectId("60d9c9ff7fde704f28e89040"),"status":"NONE","questions":[{"id":"607569f06df0a80197227d06","value":"A String Value 1"},{"id":"607569f78f403f67dd1fefed","value":"A String value 2"},{"id":"607569f78f403f67dd1fefef","value":"A String value 3"},{"id":"607569e0701e55c65eae7554","value":ObjectId("60d9c9fc7fde704f28e88fd2")},{"id":"607569e86b5b2a2175cce0f0","value":ObjectId("60d9c9fc7fde704f28e88fd2")},{"id":"607569fff2597be5ff2444fd","value":ObjectId("60d9c9fc7fde704f28e88fd4")},{"id":"607569fff2597be5ff2444fc","value":ObjectId("60d9c9fc7fde704f28e88fe4")}]}],"ex_ref_data":[{"_id":ObjectId("60d9c9fc7fde704f28e88fe4"),"type":"type_1","displayLabel":"A"},{"_id":ObjectId("60d9c9fc7fde704f28e88fd4"),"type":"type_1","displayLabel":"B"},{"_id":ObjectId("60d9c9fc7fde704f28e88fd2"),"type":"type_2","displayLabel":"C"}]}

db.products.aggregate([{$unwind:"$questions"},{$lookup:{from:"ex_ref_data",let:{questionValue:"$questions.value"},pipeline:[{$match:{$expr:{$eq:["$$questionValue","$_id"]}}},{$project:{displayLabel:1,_id:0}}],as:"questions.value"}},{$project:{"questions.id":1,"questions.value":"$questions.value.displayLabel"}},{$unwind:"$questions.value"},{$group:{_id:"$_id",questions:{$push:"$questions"}}}])