Source

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

public function get_sitemap_list() {
		$sitemaps = array();

		$providers = $this->registry->get_providers();
		/* @var WP_Sitemaps_Provider $provider */
		foreach ( $providers as $name => $provider ) {
			$sitemap_entries = $provider->get_sitemap_entries();

			// Prevent issues with array_push and empty arrays on PHP < 7.3.
			if ( ! $sitemap_entries ) {
				continue;
			}

			// Using array_push is more efficient than array_merge in a loop.
			array_push( $sitemaps, ...$sitemap_entries );
			if ( count( $sitemaps ) >= $this->max_sitemaps ) {
				break;
			}
		}

		return array_slice( $sitemaps, 0, $this->max_sitemaps, true );
	}