db={Usercar:[{parentId:"001",cars:[{_id:1,name:"bmw",color:"red"},{_id:2,name:"Ford",color:"black"},{_id:3,name:"Volkswagen",color:"black"},]}],Userfavorite:[{parentId:"001",favoriteCars:[1,3]}]}

db.Usercar.aggregate([{$lookup:{from:"Userfavorite",localField:"parentId",foreignField:"parentId",as:"favorite"}},{$set:{favorite:{$arrayElemAt:[{$concatArrays:"$favorite.favoriteCars"},0]}}},{$addFields:{favoriteCarDetails:{$filter:{input:"$cars",cond:{$in:["$$this._id","$favorite"]}}}}}])