<!-- Accessing a product metafield -->
<p>Washing: {{ product.metafields.my_app.washing_instructions }}</p>

<!-- Accessing a file/image metafield -->
{% assign file = product.metafields.my_app.size_chart.value %}
<img src="{{ file | image_url: width: 500 }}" />

<!-- Checking existence -->
{% if product.metafields.my_app.instructions != blank %}
   ...
{% endif %}

query {
  product(id: "gid://shopify/Product/123") {
    title
    metafield(namespace: "my_app", key: "instructions") {
      value
      type
    }
  }
}

mutation metafieldsSet($metafields: [MetafieldsSetInput!]!) {
  metafieldsSet(metafields: $metafields) {
    metafields {
      id
      namespace
      key
      value
    }
    userErrors {
      field
      message
    }
  }
}

/* Variables */
{
  "metafields": [
    {
      "ownerId": "gid://shopify/Product/123",
      "namespace": "my_app",
      "key": "instructions",
      "type": "single_line_text_field",
      "value": "Wash cold, tumble dry."
    }
  ]
}