Source

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

public static function get_instance( $site_id ) {
		global $wpdb;

		$site_id = (int) $site_id;
		if ( ! $site_id ) {
			return false;
		}

		$_site = wp_cache_get( $site_id, 'sites' );

		if ( false === $_site ) {
			$_site = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM {$wpdb->blogs} WHERE blog_id = %d LIMIT 1", $site_id ) );

			if ( empty( $_site ) || is_wp_error( $_site ) ) {
				$_site = -1;
			}

			wp_cache_add( $site_id, $_site, 'sites' );
		}

		if ( is_numeric( $_site ) ) {
			return false;
		}

		return new WP_Site( $_site );
	}