Your code attempt contains multiple bugs and errors:

• woocommerce_thankyou is an action hook, not a filter hook
• Only $order_id is passed to the callback function, $result is not applicable
• Use $wpdb->prefix with b03_, this can make it dynamic
• $wpdb is an object
• Use global $woocommerce, $post; is redundant

So you get:

function action_woocommerce_thankyou( $order_id ) {
    // Get $order object
    $order = wc_get_order( $order_id );
    
    // Is a WC_Order
    if ( is_a( $order, 'WC_Order' ) ) {
        // Get user id
        $user_id = $order->get_user_id();
        
        // Not a guest
        if ( $user_id > 0 ) {
            //SQL
            global $wpdb;

            // The SQL query
            $result = $wpdb->get_var( "
                SELECT SUM( oim.meta_value )
                FROM {$wpdb->prefix}woocommerce_order_itemmeta as oim
                JOIN {$wpdb->prefix}woocommerce_order_items as oi ON oim.order_item_id = oi.order_item_id
                JOIN {$wpdb->prefix}posts as p ON oi.order_id = p.ID
                JOIN {$wpdb->prefix}postmeta as pm ON p.ID = pm.post_id
                WHERE p.post_type = 'shop_order'
                AND oim.meta_key = 'trees_planted'
                AND pm.meta_value = '$user_id'
                AND pm.meta_key = '_customer_user'
                AND p.ID = '$order_id'
            " );

            // REMOVE THIS, only for testing purposes
            $result = 10;

            // Add the meta data
            $order->update_meta_data( 'trees', $result );
            $order->save();
        }
    }
}
add_action( 'woocommerce_thankyou', 'action_woocommerce_thankyou', 10, 1 );

Note: Since you are using a custom SQL query where the data/results don't exist generally/by default in WooCommerce, but just for you, I have replaced it with mine The answer has a fixed value of 10. Adjust as needed!