SELECT product_id, json_agg(json_build_object(
      'style_id', id,
      'name', name,
      'original_price', original_price,
      'sale_price', sale_price,
      'default?', default_style,
      'photos',

// use your json_agg inside of another json_agg with parentheses!

      (SELECT json_agg(json_build_object(
          'thumbnail_url', thumbnail_id,
          'url', url
        )) FROM photos WHERE style_id = styles.id),
    'skus',

// Object in object can be also used using parentheses!!

      (SELECT
          json_object_agg(id,
              json_build_object(
            'size', size,
            'quantity', quantity
              )
          ) as skus
        FROM skus
        WHERE style_id = styles.id
            GROUP by style_id)
    )) as results FROM styles
        WHERE styles.product_id = 1
          GROUP BY product_id