[{"_id":"61922aed85c74b2d1ef671bb","employee":{"firstName":"fname","lastName":"lname","middleName":"mname"},"days":[{"_id":"61922aed85c74b2d1ef671be","day":"2021-09-01T21:00:00.000Z","data":{"hours":0,"type":"OT","_id":"61922aed85c74b2d1ef671bf","updateDate":"2021-11-15T09:39:57.624Z"}},{"_id":"61922aed85c74b2d1ef671c0","day":"2021-09-02T21:00:00.000Z","data":{"hours":0,"type":"OT","_id":"61922aed85c74b2d1ef671c1","updateDate":"2021-11-15T09:39:57.625Z"}}]}]

db.collection.aggregate([{"$project":{"_id":1,"employee":1,"days":{"$map":{"input":"$days","as":"d","in":{"_id":"$$d._id","day":"$$d.day","hours":"$$d.data.hours","type":"$$d.data.type","updateData":"$$d.data.updateDate"}}}}}])