Enrollment is different from authentication. In authentication, you grant a user access to your site. In enrollment, you grant a user access to a course.
Moodle offers a variety of ways to enroll users.
Administration → Site Administration → Plugins → Enrolments → Manage Enrol Plugins
The number of Instances tells you how many courses have this enrollment method added to them. However, just because a course has an enrollment method added to the course, doesn't mean that course is using the enrollment method.
Manual enrollment is the default form of enrollment. When this is selected, a teacher or administrator can enroll the student.
Moodle site has a special user called "Guest". This user can be granted access to courses, without requiring to be enrolled. You are allowing anonymous users to access your site and course.
If you require a password for guest access, you'll need to tell your guests what the password is. You could add that to the course description that the visitors see on the front page of your site.
This allows users to enroll themselves in courses. As with guest access, you must enable this method under Site administration for your entire site, and also activate it for a specific course.
In a Moodle course, you can add students to a group. Then, you can manage that group's access to activities and resources in the course instead of managing the students one at a time. Moodle also enables you to create site-wide groups. These are called cohorts.
The cohorts sync enrollment method enables you to enroll a cohort into a course. This enrolls everyone in that cohort in one go.
A flat file is a text file that contains information from a database. The flat file method of student enrollment causes Moodle to read in a text file, and use that as the source for enrollment information. The flat file method is especially useful if you need to enroll a large group of people who have records in another system.
An IMS Enterprise File is a flat file (text file) that conforms to standards set by the IMS Global Learning Consortium. Many student information systems and human resources information systems can export an IMS-compliant file. For example, PeopleSoft and Oracle can export IMS files.
You can use an external database to control student enrollment. In this case, Moodle looks in the designated database and determines if the student is enrolled.
The PayPal option enables you to set up paid access to the site, or to individual courses. When you select this option, you enter a value into the field Enrol Cost. This becomes the fee for joining the site. If you enter zero into enrol_cost, students can access the site for free. If you enter a non-zero amount, students must pay to access the site.