How to display specific K2 extra field of current item with k2 tool module

Start by extracting the extra_fields column's json string from #__k2_items table at the row of your choosing, then decode it. Each row from the array (generated by decoding json), will have two elements having respective keys: id and value .

I don't quite know the full picture of what you are trying to do, but I can help you to relate and extract the data.

$output = [];
$db = JFactory::getDBO();
$db->setQuery('SELECT extra_fields FROM #__k2_items WHERE id = ' . JRequest::getInt('id'));
foreach (json_decode($db->loadResult() ?: '[]', true) as ['id' => $id, 'value' => $value]) {
    
    // Access the extra_fields using this row's id
    $db->setQuery('SELECT * FROM #__k2_extra_fields WHERE id = ' . (int)$id);
    $row = $db->loadAssoc();
    $output[] = [
        'id' => $id,
        'value' => $value,
        'extraFieldName' => $row['name'],
        'extraFieldType' => $row['type'],
        'decoded' => json_decode($row['value'], true)
    ];
}
var_export($output);

This will effectively display the following:

[
    [
        'id' => '1',
        'value' => 'PERKINS',
        'extraFieldName' => 'BRAND',
        'extraFieldType' => 'textfield',
        'decoded' => [
            [
                'name' => null,
                'value' => '',
                'target' => null,
                'alias' => 'brand',
                'required' => 1,
                'showNull' => 0
            ]
        ]
    ],
    [
        'id' => '2',
        'value' => '14',
        // more data that I can't see...
    ],
]

I have seen snippets in K2 such as $item->extra_fields = $model->getItemExtraFields($item->extra_fields, $item); which indicate that there are some pre-existing helpful methods on offer, but I must admit that I am not a K2 user, so I am poorly suited to advise on these.