db={Orders:[{_id:"ord1",items:[{catalog_id:"xyz1",qty:5},{catalog_id:"xyz2",qty:3}]}],Catalogs:[{_id:"xyz1",name:"my catalog name",SKU:"XxYxZx1"},{_id:"xyz2",name:"my other catalog name",SKU:"XxYxZx2"}]}

db.Orders.aggregate([{$lookup:{from:"Catalogs",let:{"catalogId":"$items.catalog_id"},pipeline:[{$match:{$expr:{$in:["$_id","$$catalogId"]}}},{$project:{"name":1,"merchant_SKU":1}}],as:"items_ex"}}])