[{"_id":1,"items":[{"createdAt":"2024-04-08T14:33:56.734Z","id":"6614005475802af9ae05b6b2","position":0,"title":"title"},{"createdAt":"2024-04-08T14:22:30.880Z","id":"6613fda792ffb86a14d266ea","position":1,"title":"testing"},{"createdAt":"2024-04-08T14:24:54.580Z","id":"6613fe3675802af9ae05b6a2","position":2,"title":"cool stuff"}]}]

db.collection.update({_id:1},[{$set:{"items":{$concatArrays:["$items",[{"id":"new ID here","createdAt":"2024-04-09T01:23:45.678Z","title":"more cool stuff","position":{$add:[{$max:"$items.position"},1]}}]]}}}])