Database
[{_id:2,"innerArray":[{"originalDateTime":ISODate("2022-01-02T01:01:01Z")}]}]
Query
db.collection.update({},[{"$set":{"innerArray":{$map:{input:"$innerArray",in:{$mergeObjects:["$$this",{copiedDateTime:"$$this.originalDateTime"}]}}}}}],{"multi":false,"upsert":false})