Adding a custom field

There are different types of fields you can add for use with different product types and you can display your custom field content using different single product page hook locations.

This code adds a text field to simple products.

Use the code in your child themes functions file or code snippets plugin.

// Add custom text field to backend (simple products only)
add_action( 'woocommerce_product_options_general_product_data', 'bd_add_custom_backend_text_field' );
function bd_add_custom_backend_text_field() {
    global $product_object;

    if ( $product_object && $product_object->is_type( 'simple' ) ) {
        echo '<div class="options_group">';
        woocommerce_wp_text_input( [
            'id'          => '_bd_custom_text',
            'label'       => __( 'Custom Text Field', 'woocommerce' ),
            'desc_tip'    => true,
            'description' => __( 'Enter a custom value for this simple product.', 'woocommerce' ),
        ] );
        echo '</div>';
    }
}

// Save the custom field value
add_action( 'woocommerce_admin_process_product_object', 'bd_save_custom_backend_text_field' );
function bd_save_custom_backend_text_field( $product ) {
    if ( isset( $_POST['_bd_custom_text'] ) ) {
        $product->update_meta_data( '_bd_custom_text', sanitize_text_field( $_POST['_bd_custom_text'] ) );
    }
}

// Output custom fields on the frontend product page
add_action( 'woocommerce_single_product_summary', 'bd_output_custom_product_fields' );
function bd_output_custom_product_fields() {

    global $product;

    // Only for simple products
    if ( ! $product->is_type( 'simple' ) ) return;

    // Get field values
    $text     = get_post_meta( $product->get_id(), '_bd_custom_text', true );

    echo '<h3>Product Details</h3>';

    if ( $text ) {
        echo '<div><strong>Text:</strong> ' . esc_html( $text ) . '</div>';
    }
}

Tested and works.