[{_id:123456,brdate:"10-08-2022",brName:"Principal Branch, Dhaka",email:"example_email",loans:[[{loan_id:1,loan_name:"X",category:"rural"},{loan_id:2,loan_name:"Y",category:"urban"},{loan_id:3,loan_name:"Z",category:"rural"}],[{loan_id:4,loan_name:"A",category:"rural"},{loan_id:5,loan_name:"B",category:"urban"}],]}]

db.collection.aggregate([{$match:{_id:123456}},{$unwind:"$loans"},{$unwind:"$loans"},{$group:{_id:"$loans.category",loanCategory:{$addToSet:"$loans"}}}])