[{"_id":{"$oid":"64ddcb04756827cb9d4633ba"},"site":"site1","group":"env1","items":{"x":{"release":"true"},"y":{"release":"true"},"z":{"release":"true"}}},{"_id":{"$oid":"64ddcb04756827cb9d4633cd"},"site":"site2","group":"env1","items":{"x":{"release":"true"},"y":{"release":"true"},"z":{"release":"true"}}}]

db.collection.aggregate([{$group:{_id:"$group",siteitems:{$push:{$map:{input:{$objectToArray:"$items"},as:"item",in:{$concat:["$site","$$item.k"]}}}}}},{$set:{siteitems:{$reduce:{input:"$siteitems",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}}])