db={"missions":[{"_id":ObjectId("6045eec8f113547ddd3472d9"),"execution":{"_id":ObjectId("6045eec8f113547ddd3472d8"),"steps":[{"_id":ObjectId("6045eec8f113547ddd3472d7"),"actions":[{"_id":ObjectId("6045eec8f113547ddd3472d6"),"title":"action title"},{"_id":ObjectId("6045eec8f113547ddd3472da"),"title":"action title 2"}]}]}}]}

db.missions.aggregate([{$match:{_id:ObjectId("6045eec8f113547ddd3472d9")}},{$addFields:{"allActions":{"$reduce":{"input":"$execution.steps.actions","initialValue":[],"in":{"$concatArrays":["$$value","$$this"]}}}}}])