How to query json stored as string in bigquery table?

How to query json stored as string in bigquery table? I have a table where value in a column (subscriptions) looks like this:

{
    "data": [{
        "application_fee_percent": null,
        "canceled_at": null,
        "created": 1500476240,
        "items": {
            "data": [{
                "created": 1500476240,
                "id": "s4nQMWJn4P1Lg",
                "metadata": {},
                "object": "subscription_item",
                "plan": {
                    "amount": 3,
                    "created": 1494270926,
                    "currency": "usd",
                    "livemode": true,
                    "metadata": {
                        "currentlySelling": "true",
                        "features": "{\"shipping\": true,\"transactionFee\":0.00}",
                        "marketingFeatures": "[\"Unlimited products\"]"
                    },
                    "name": "Personal",
                    "object": "plan",
                    "statement_descriptor": null,
                    "trial_period_days": null
                },
                "quantity": 1
            }],
            "has_more": false,
            "object": "list",
            "total_count": 1,
            "url": "/v1/subscri3XwjA3"
        },
        "livemode": true,
        "metadata": {
            "test": "596f735756976"
        },
        "object": "suion",
        "quantity": 1
    }],
    "has_more": false,
    "object": "list",
    "total_count": 1,
    "url": "/v1/cutions"
}

How can I select application_fee_percent, features, and marketingFeatures?

For created, I tried JSON_EXTRACT("subscriptions", "$.data[0].created") but it's returning null.