db={"collection":[{id:1,"total":5},{id:2,"total":2}],"collection2":[{collId:1,"total":3},{collId:2,"total":4}]}

db.collection.aggregate([{$lookup:{from:"collection2",let:{col_id:"$id",col_total:"$total",},pipeline:[{$match:{$expr:{$eq:["$collId","$$col_id"]}}},{$project:{summary:{$multiply:["$total","$$col_total"]}}}],as:"result"}},{$addFields:{result:{$let:{vars:{tmp:{$arrayElemAt:["$result",0]}},in:"$$tmp.summary"}}}}])