[{"_id":"ddd00000000000000cb00001","title":"Title","questions":[{"_id":"60bfc1afcd3d6b68d44b1e23"},{"_id":"60bfc1afcd3d6b68d44b1e27"},{"_id":"60bfc1afcd3d6b68d44b1e2b"}]}]

db.collection.aggregate([{$match:{}},{$limit:1},{$addFields:{questions:"$questions._id"}}])