How to display custom field in product description?

You can take the help of hooks WooCommerce provides. Here is the link to the visual hook guide https://www.businessbloomer.com/woocommerce-visual-hook-guide-single-product-page/

Here is the code that suits your need.

add_action( 'woocommerce_after_single_product_summary', function() {
    global $product;

    if ( $product->is_type( 'simple' ) ) {
        get_post_meta( get_the_ID(), '_additional_description' );
    }
} );

I hope it helps.