db={"seller":[{name:"String",url:"abc.com"},{name:"somename",url:"xyz.com"}],"product":[{title:"bike",sellerUrl:"abc.com"},{title:"car",sellerUrl:"abc.com"},{title:"truck",sellerUrl:"def.com"},{title:"db",sellerUrl:"mongo.com"}]}

db.product.aggregate([{$group:{_id:"",sellerUrls:{$addToSet:"$sellerUrl"}}},{$lookup:{from:"seller",let:{sellerUrls:"$sellerUrls"},pipeline:[{$group:{_id:"",urls:{$addToSet:"$url"}}},{$project:{_id:0,uniqueAndNotInSellerColl:{$setDifference:["$$sellerUrls","$urls"]}}}],as:"data"}},{$replaceRoot:{newRoot:{$arrayElemAt:["$data",0]}}}])