Courses, lessons, quizzes and student progress are managed here.
Enrollment is what grants access
Access to a course comes from an enrollment, not directly from a purchase. A purchase usually creates one, but a manual enrollment is equally valid — for free courses, internal access, or anything paid for outside the system.
When someone has paid and cannot see the course, the question to ask is whether their enrollment exists.
A locked lesson is not always a fault
A lesson can be closed deliberately: either scheduled to open a number of days after enrollment, or gated behind a prerequisite the student has not finished.
To the student both look the same. The course's own settings are what distinguish them.
Quiz answers stay on the server
Correct answers are never sent to the browser with the questions, and grading happens server-side. They cannot be pulled out of the page.
The practical consequence is that editing a question after someone has taken the quiz does not rewrite their existing score.
Progress is the sum of what was completed
The progress percentage is derived from what a student has actually done. Adding a new lesson to a course someone is taking lowers their percentage — because the denominator grew, not because anything was lost.