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