Database

[{_id:1,parentId:1,"stats":{"type":{"a":{"n:1":2,"n:2":2},"b":{"n:1":2,"n:2":1,"n:3":1},"c":{"n:5":4}}},time:ISODate("2021-10-12T05:00:00Z")},{_id:2,parentId:1,"stats":{"type":{"a":{"n:1":1,},"b":{"n:1":2,"n:2":3,"n:3":4},"c":{"n:4":2}}},time:ISODate("2021-10-12T06:00:00Z")},{_id:3,parentId:2,"stats":{"type":{"a":{"n:1":3,},"b":{"n:2":5,"n:3":7},"c":{"n:1":1,"n:5":2}}},time:ISODate("2021-10-13T05:00:00Z")}]

Query

db.collection.aggregate([{$set:{stats:{$objectToArray:"$stats.type"}}},{$set:{stats:{$map:{input:"$stats.v",in:{$objectToArray:"$$this"}}}}},{$set:{stats:{$reduce:{input:"$stats",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}},{$unwind:"$stats"},{$group:{_id:{parentId:"$parentId",day:{$dateTrunc:{date:"$time",unit:"day"}},k:"$stats.k"},v:{$sum:"$stats.v"}}},{$group:{_id:{parentId:"$_id.parentId",day:"$_id.day"},stats:{$push:{k:"$_id.k",v:"$v"}}}},{$set:{stats:{$arrayToObject:"$stats"}}},{$set:{"_id.day":{$dateToParts:{date:"$_id.day"}}}},{$replaceRoot:{newRoot:{$mergeObjects:["$_id","$stats"]}}},{$set:{year:"$day.year",month:"$day.month",day:"$day.day"}}])

Result