What is the best way to make some content only available to authenticated users?
You can use the combination of ACL and Content access modules to achieve this. ACL + Content access give you better control over access to content: You can make certain types accessible to certain roles, or even specific nodes accessible to specific users.

Now here comes the tricky part:

Both the Acl and Content Access modules have no D8 release yet and have patches in their issue queues, which convert them to D8. For some reason the maintainers have not created D8 branches yet, but I'm using the patched versions of both the modules on my company site - they work perfectly.

ACL patch can be found here.

Content access patch can be found here.