Source

File: wp-includes/class-wp-network.php

private function get_main_site_id() {
		/**
		 * Filters the main site ID.
		 *
		 * Returning a positive integer will effectively short-circuit the function.
		 *
		 * @since 4.9.0
		 *
		 * @param int|null   $main_site_id If a positive integer is returned, it is interpreted as the main site ID.
		 * @param WP_Network $network      The network object for which the main site was detected.
		 */
		$main_site_id = (int) apply_filters( 'pre_get_main_site_id', null, $this );
		if ( 0 < $main_site_id ) {
			return $main_site_id;
		}

		if ( 0 < (int) $this->blog_id ) {
			return (int) $this->blog_id;
		}

		if ( ( defined( 'DOMAIN_CURRENT_SITE' ) && defined( 'PATH_CURRENT_SITE' ) && DOMAIN_CURRENT_SITE === $this->domain && PATH_CURRENT_SITE === $this->path )
			|| ( defined( 'SITE_ID_CURRENT_SITE' ) && SITE_ID_CURRENT_SITE == $this->id ) ) {
			if ( defined( 'BLOG_ID_CURRENT_SITE' ) ) {
				$this->blog_id = (string) BLOG_ID_CURRENT_SITE;

				return (int) $this->blog_id;
			}

			if ( defined( 'BLOGID_CURRENT_SITE' ) ) { // Deprecated.
				$this->blog_id = (string) BLOGID_CURRENT_SITE;

				return (int) $this->blog_id;
			}
		}

		$site = get_site();
		if ( $site->domain === $this->domain && $site->path === $this->path ) {
			$main_site_id = (int) $site->id;
		} else {
			$cache_key = 'network:' . $this->id . ':main_site';

			$main_site_id = wp_cache_get( $cache_key, 'site-options' );
			if ( false === $main_site_id ) {
				$_sites       = get_sites(
					array(
						'fields'     => 'ids',
						'number'     => 1,
						'domain'     => $this->domain,
						'path'       => $this->path,
						'network_id' => $this->id,
					)
				);
				$main_site_id = ! empty( $_sites ) ? array_shift( $_sites ) : 0;

				wp_cache_add( $cache_key, $main_site_id, 'site-options' );
			}
		}

		$this->blog_id = (string) $main_site_id;

		return (int) $this->blog_id;
	}