Database
[{"_id":"101","animalArray":[["dog"],["lizard","lion"]],"array_b":[10,20]}]
Query
db.collection.aggregate([{$project:{"animalArray":{$concatArrays:[[{$arrayElemAt:["$animalArray",0]}],[{$concatArrays:[{$arrayElemAt:["$animalArray",1]},"$array_b"]}],]}}}])