Database
[{"_id":"101","animalArray":[["dog"],["lizard","lion"],["optional"]],"array_b":[10,20]}]
Query
db.collection.aggregate([{$project:{animalArray:1,mergedArray:{$reduce:{input:"$array_b",initialValue:{$arrayElemAt:["$animalArray",1]},in:{$concatArrays:["$$value",["$$this"]]}}}}},{$project:{animalArray:{$concatArrays:[[{$arrayElemAt:["$animalArray",0]}],["$mergedArray"]]}}}])