How to change wordpress RSS to be in a specific format?

You can use the action rss2_item (and rss_item for RSS 0.92) to output the required tags:

function wpse_406826_rss_featured_image() {
    $src = get_the_post_thumbnail_url( null, 'medium' /* or whatever size you need */ );

    if ( $src ) {
        echo "<image><url>$src</url></image>";
    }
}

add_action( 'rss2_item', 'wpse_406826_rss_featured_image' );
add_action( 'rss_item',  'wpse_406826_rss_featured_image' );