[{_id:ObjectId("6188a5283543f7cc2f77c73f"),branch_id:1,borrowers_list:{0:{borrowed_dt:ISODate("2021-08-15T06:00:00.000+00:00"),card_no:"ID000067",return_dt:ISODate("2021-08-25T06:00:00.000+00:00")}}}]

db.collection.aggregate([{$set:{borrowers:{"$objectToArray":"$borrowers_list"}}},{$set:{Date_diff:{$map:{input:"$borrowers",as:"b",in:{k:"$$b.k",v:{$dateDiff:{startDate:"$$b.v.borrowed_dt",endDate:"$$b.v.return_dt",unit:"day"}}}}}}},{$project:{Date_diff:{"$arrayToObject":"$Date_diff"}}}])