[{"year":2020,"cod":4241,"op":"12"},{"year":2021,"cod":2241,"op":"34"},{"year":2020,"cod":4325,"op":"54"},{"year":2021,"cod":1132,"op":"A3"}]

db.collection.aggregate([{$group:{_id:"$year",values:{$push:{cod:"$cod",op:"$op"}}}},{$project:{year:"$_id",values:1,_id:0}}])