Database
[{"_id":"101","animalArray":[["dog"],["lizard","lion"],["optional"]],"array_b":[10,20]}]
Query
db.collection.aggregate([{$project:{animalArray:{$map:{input:"$animalArray",in:{$cond:[{$eq:["$$this",{$arrayElemAt:["$animalArray",1]}]},{$reduce:{input:"$array_b",initialValue:"$$this",in:{$concatArrays:["$$value",["$$this"]]}}},"$$this"]}}}}}])