Courses and the structure of their lessons.
Publishing does not grant access
Publishing means the course appears in the public listing and can be bought. Its content stays closed.
What opens the lessons is enrolment. So a course can be live on the site with nobody able to see inside it — and that is it working correctly.
The structure has three layers
A course is made of modules, and each module of lessons. The order is set here and it is the order the student sees.
A lesson that belongs to no module never appears in the student's path.
Lessons can be locked on purpose
A lesson can be gated on time since enrolment, or on completing another lesson. The student then sees the lesson but cannot open it.
So "a student says a lesson will not open" is, before it is a fault, usually the setting you put there.
Lesson content is written in the Studio
Lessons are built with the same page-building tool, not a plain text editor. Anything you can put on a page can go in a lesson.