Help with join documents

Working with Data Drivers
1

Hi, I’m really newby and apology for the stupid question.
I have two douments and I need to do a left join counting the right elements.
Let me explain:
this is the first document

{ 
   "id": "string"
 , "matches": [
            { "timeMatch": "string"
              , "matchID": "string"
             , "opponentName": "string"
             , "playerPoint": 0
            , "opponentPoint": 0 } 
           ]
   }

This is the second

{  "matchID": "string",  "boards": [ {
  "round_mum": 0,
  "board": [
    {
      "hexName": "string",
      "who": "string",
      "round": 0,
      "score": 0,
      "multiplier": 0,
      "color": 0,
      "status": 0
    }
  ]
}]}

I need to have all lines of the first document putting the aggregate information from the second mantaining onli the MatchID and sum of board
The possible result could be

"id": "AAAA",
"matches" : [
              id: "BBBBB"    , board : 2
             ,id: "CCCCC"    , board : 4
             ,id: "DDDDD"    , board : 6
]

I use c# and I tryed do this kind of aggregation

var docs = FirstCollection.Aggregate()
                    .Lookup("SecondCollection", "MatchID", "id", @as: "match")
                    .Unwind("match")
                    .As<BsonDocument>()
                    .ToEnumerable();

and I don’t know hot to group the second counting the boards

Someone can help me?