[{fieldA:[{_id:1,value:1},{_id:2,value:2},{_id:3,value:3}],fieldB:[{_id:2},{_id:3},{_id:4},],}]

db.collection.aggregate([{"$addFields":{"fieldC":{"$filter":{"input":"$fieldA","as":"a","cond":{"$in":["$$a._id","$fieldB._id"]}}}}}])