Source

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

public function get_sitemap_url( $name, $page ) {
		global $wp_rewrite;

		// Accounts for cases where name is not included, ex: sitemaps-users-1.xml.
		$params = array_filter(
			array(
				'sitemap'         => $this->name,
				'sitemap-subtype' => $name,
				'paged'           => $page,
			)
		);

		$basename = sprintf(
			'/wp-sitemap-%1$s.xml',
			implode( '-', $params )
		);

		if ( ! $wp_rewrite->using_permalinks() ) {
			$basename = '/?' . http_build_query( $params, null, '&' );
		}

		return home_url( $basename );
	}