To make your site completely private (intranet or full private content platform), you have to install and activate the My Private Site plugin.
After that go to the plugin setting page (Settings > Private Site) and adjust options according to your needs.

Here is a link of a well-written article about this subject :
https://www.wpbeginner.com/beginners-guide/how-to-make-your-wordpress-blog-completely-private/

For more information about the plugin options, we invite you to read the author FAQ here : https://wordpress.org/plugins/jonradio-private-site/