db={"vehicles":[{"_id":1,brand:"",model:"",year:"",suppliers:[{"name":"","contact":"","supplierId":"5a934e000102030405000001","orders":[],"info":""}]}],"suppliers":[{"name":"","contact":"","_id":ObjectId("5a934e000102030405000001"),"internalId":"2","address":""}]}

db.vehicles.aggregate([{$lookup:{from:"suppliers",let:{supplierId:"$suppliers.supplierId"},pipeline:[{$match:{$expr:{$in:[{$toString:"$_id"},"$$supplierId"]}}},{$project:{_id:0,supplierId:{$toString:"$_id"},internalId:1}}],as:"suppliers_data"}},{$project:{suppliers:{$map:{input:"$suppliers",as:"s",in:{$mergeObjects:["$$s",{internalId:{$reduce:{input:"$suppliers_data",initialValue:"",in:{$cond:[{$eq:["$$this.supplierId","$$s.supplierId"]},"$$this.internalId","$$value"]}}}}]}}}}}])