wp_initialize_site

 *     @type array  $options Custom option $key => $value pairs to use. Default
 *                           empty array.
 *     @type array  $meta    Custom site metadata $key => $value pairs to use.
 *                           Default empty array.
 * }
 * @return bool|WP_Error True on success, or error object on failure.
 */
function wp_initialize_site( $site_id, array $args = array() ) {
	global $wpdb, $wp_roles;

	if ( empty( $site_id ) ) {
		return new WP_Error( 'site_empty_id', __( 'Site ID must not be empty.' ) );
	}

	$site = get_site( $site_id );
	if ( ! $site ) {
		return new WP_Error( 'site_invalid_id', __( 'Site with the ID does not exist.' ) );
	}

	if ( wp_is_site_initialized( $site ) ) {
		return new WP_Error( 'site_already_initialized', __( 'The site appears to be already initialized.' ) );
	}

	$network = get_network( $site->network_id );
	if ( ! $network ) {
		$network = get_network();
	}

	$args = wp_parse_args(
		$args,
		array(
			'user_id' => 0,
			/* translators: %d: Site ID. */
			'title'   => sprintf( __( 'Site %d' ), $site->id ),
			'options' => array(),
			'meta'    => array(),
		)
	);

	/**
	 * Filters the arguments for initializing a site.
	 *
	 * @since 5.1.0
	 *
	 * @param array      $args    Arguments to modify the initialization behavior.
	 * @param WP_Site    $site    Site that is being initialized.
	 * @param WP_Network $network Network that the site belongs to.
	 */
	$args = apply_filters( 'wp_initialize_site_args', $args, $site, $network );

	$orig_installing = wp_installing();
	if ( ! $orig_installing ) {
		wp_installing( true );
	}

	$switch = false;
	if ( get_current_blog_id() !== $site->id ) {
		$switch = true;
		switch_to_blog( $site->id );
	}

	require_once ABSPATH . 'wp-admin/includes/upgrade.php';

	// Set up the database tables.
	make_db_current_silent( 'blog' );

	$home_scheme    = 'http';
	$siteurl_scheme = 'http';
	if ( ! is_subdomain_install() ) {
		if ( 'https' === parse_url( get_home_url( $network->site_id ), PHP_URL_SCHEME ) ) {
			$home_scheme = 'https';
		}
		if ( 'https' === parse_url( get_network_option( $network->id, 'siteurl' ), PHP_URL_SCHEME ) ) {
			$siteurl_scheme = 'https';
		}
	}

	// Populate the site's options.
	populate_options(
		array_merge(
			array(
				'home'        => untrailingslashit( $home_scheme . '://' . $site->domain . $site->path ),
				'siteurl'     => untrailingslashit( $siteurl_scheme . '://' . $site->domain . $site->path ),
				'blogname'    => wp_unslash( $args['title'] ),
				'admin_email' => '',
				'upload_path' => get_network_option( $network->id, 'ms_files_rewriting' ) ? UPLOADBLOGSDIR . "/{$site->id}/files" : get_blog_option( $network->site_id, 'upload_path' ),
				'blog_public' => (int) $site->public,
				'WPLANG'      => get_network_option( $network->id, 'WPLANG' ),
			),
			$args['options']
		)
	);

	// Clean blog cache after populating options.
	clean_blog_cache( $site );

	// Populate the site's roles.
	populate_roles();
	$wp_roles = new WP_Roles();

	// Populate metadata for the site.
	populate_site_meta( $site->id, $args['meta'] );

	// Remove all permissions that may exist for the site.
	$table_prefix = $wpdb->get_blog_prefix();
	delete_metadata( 'user', 0, $table_prefix . 'user_level', null, true );   // Delete all.
	delete_metadata( 'user', 0, $table_prefix . 'capabilities', null, true ); // Delete all.

	// Install default site content.
	wp_install_defaults( $args['user_id'] );

	// Set the site administrator.
	add_user_to_blog( $site->id, $args['user_id'], 'administrator' );
	if ( ! user_can( $args['user_id'], 'manage_network' ) && ! get_user_meta( $args['user_id'], 'primary_blog', true ) ) {
		update_user_meta( $args['user_id'], 'primary_blog', $site->id );
	}

	if ( $switch ) {