[{"_id":"662aa6ccae109745e30dc664","username":"username","email":"email@email.com","lists":[{"_id":"662aa6dbae109745e30dc66a","name":"list","content":[{"_id":"662aa6eeae109745e30dc670","name":"product","quantity":30,"brands":[]},{"_id":"662aa6eeae109745e30dffff","name":"OTHER product","quantity":100,"brands":[]}]},{"_id":"662aa6dbae109745e30deeee","name":"OTHER list","content":[{"_id":"662aa6eeae109745e30ddddd","name":"new product","quantity":5,"brands":[]}]}]}]

db.collection.aggregate([{$unwind:"$lists"},{$unwind:"$lists.content"},{$match:{"lists.content._id":"662aa6eeae109745e30dc670"}},{$project:{"lists._id":1,"lists.content._id":1}},/** limit to just one doc if that's a requirement; NOT recommended*/{$limit:1}])