How to allow enrollment in courses?

I've created two dummy courses and when I'm logged in there is an "enroll" button but not when I'm logged out.

I have the courses set to "registered users" as there is no option for unregistered- but how do people even get registered when that option isn't listed for them?

I would think they would buy the course and then get a registration confirmation email with info on how to login and create their credentials, right?

How does this process work for new users?