Site under construction? Unlike many other such plugins, which create beautiful pages with countdowns, shovels and whatnot, this one redirects unregistered users to a page of your choosing. Magic links allow you to create exceptions for your client, family or friends!
Not logged in users can't see your site and are redirected to whatever page you want. Can be a static page on your server, an old site. By default it redirects to a very simple Under Construction page. Configure the URL in the "Under Construction" page under "Settings" menu. To enable your site back deactivate "Under Construction" plugin.
Magic links permit someone to gain "preview mode" to the site. The "Under Construction" page under "Settings" menu has a section where keys can created and deleted. A note can be associated with the key: it can help you remember the person you sent the key to. Under each key the "Copy to clipboard" allows you to copy a link that you can send to the person you want to see the site. This works by creating a session cookie.