Source

File: wp-includes/sitemaps/class-wp-sitemaps-renderer.php

public function get_sitemap_index_xml( $sitemaps ) {
		$sitemap_index = new SimpleXMLElement(
			sprintf(
				'%1$s%2$s%3$s',
				'<?xml version="1.0" encoding="UTF-8" ?>',
				$this->stylesheet_index,
				'<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" />'
			)
		);

		foreach ( $sitemaps as $entry ) {
			$sitemap = $sitemap_index->addChild( 'sitemap' );

			// Add each element as a child node to the <sitemap> entry.
			foreach ( $entry as $name => $value ) {
				if ( 'loc' === $name ) {
					$sitemap->addChild( $name, esc_url( $value ) );
				} elseif ( 'lastmod' === $name ) {
					$sitemap->addChild( $name, esc_xml( $value ) );
				} else {
					_doing_it_wrong(
						__METHOD__,
						sprintf(
							/* translators: %s: List of element names. */
							__( 'Fields other than %s are not currently supported for the sitemap index.' ),
							implode( ',', array( 'loc', 'lastmod' ) )
						),
						'5.5.0'
					);
				}
			}
		}

		return $sitemap_index->asXML();
	}