How do I restrict access or require a login to view content?

You don’t need a plugin to restrict individual pages as that functionality is built into WordPress. Just go to Pages in the Admin Dashboard and tick the pages you want to restrict. Then in the “Bulk Actions” dropdown select “Edit” and update Status to Private. That’s it, and it also works for Posts.

However, if you want to restrict more than just a few posts or pages, then a plugin is recommended.

My plugin doesn't offer the ability to restrict access.  However, there's a couple of other plugins on the market that do:

miniOrange:  https://plugins.miniorange.com/wordpress-media-restriction

Here's a screencast on miniOrange:

PDA Gold:  https://preventdirectaccess.com/

I haven't totally vetted PDA Gold's solution yet as I couldn't get them to send me a test plugin.  However, I have used miniOrange's product and it works very well with my plugin.