Database
[{"SitesList":[["553550","496573","496574"],["553550","496573","496574"],["527772","565085"],["565085","563248"],["496576"]]}]
Query
db.collection.aggregate([{$addFields:{SitesList:{$substr:[{$reduce:{input:{$reduce:{input:"$SitesList",initialValue:[],in:{$concatArrays:["$$this","$$value"]}}},initialValue:"",in:{$concat:["$$value",",","$$this"]}}},1,-1]}}}])