How does indexing works for $lookup

How do indexing works after $lookup pipeline and that to on the data received after lookup. For ex:

    [
        {
            "$lookup": {
                "from": "order_products",
                "let": {
                    "order_id": "$order_id"
                },
                "pipeline": [
                    {
                        "$match": {
                            "$expr": {
                                "$and": [
                                    {
                                        "$eq": [
                                            "$order_id",
                                            "$$order_id"
                                        ]
                                    },                     
                                ]
                            }
                        }
                    }
                ],
                "as": "order_product"
            }
        },
        {
            "$unwind": {
                "path": "$order_product",
                "preserveNullAndEmptyArrays": true
            }
        }```
Now if i want numeric sorting on 'order_product.<fieldname>' will indexing play itz role or not? and what the better approach to get indexing for the lookup result set (keeping the execution timing main concern)

Please read Formatting code and log snippets in posts and re-post your pipeline so that it is easier to understand.