An example of how robots.txt file content can be altered.

/**
 * Add Disallow for some file types.
 * Add "Disallow: /wp-login.php/\n".
 * Remove "Allow: /wp-admin/admin-ajax.php\n".
 * Calculate and add a "Sitemap:" link.
 */
add_filter( 'robots_txt', function( $output, $public ) {
	/**
	 * If "Search engine visibility" is disabled,
	 * strongly tell all robots to go away.
	 */
	if ( '0' == $public ) {

		$output = "User-agent: *\nDisallow: /\nDisallow: /*\nDisallow: /*?\n";

	} else {

		/**
		 * Get site path.
		 */
		$site_url = parse_url( site_url() );
		$path	  = ( ! empty( $site_url[ 'path' ] ) ) ? $site_url[ 'path' ] : '';

		/**
		 * Add new disallow.
		 */
		$output .= "Disallow: $path/wp-login.php\n";

		/**
		 * Disallow some file types
		 */
		foreach(['jpeg','jpg','gif','png','mp4','webm','woff','woff2','ttf','eot'] as $ext){
			$output .= "Disallow: /*.{$ext}$\n";
		}

		/**
		 * Remove line that allows robots to access AJAX interface.
		 */
		$robots = preg_replace( '/Allow: [^\0\s]*\/wp-admin\/admin-ajax\.php\n/', '', $output );

		/**
		 * If no error occurred, replace $output with modified value.
		 */
		if ( null !== $robots ) {
			$output = $robots;
		}
		/**
		 * Calculate and add a "Sitemap:" link.
		 * Modify as needed.
		 */
		$output .= "Sitemap: {$site_url[ 'scheme' ]}://{$site_url[ 'host' ]}/sitemap_index.xml\n";
	}

	return $output;

}, 99, 2 );  // Priority 99, Number of Arguments 2.