A lesson is built out of pieces - words, pictures, questions with somewhere to write the answer. The editor is where you add, remove and rearrange them, and nothing changes for a reader until you save.
Open Library, then Browse, and walk down to the lesson.
Press the lesson to open the Lesson Editor.
Build the lesson in the panel below the heading.
Press Save Lesson.
The version number beside the title goes up each time you save, and a draft lesson - one fresh from an import - becomes live when it is saved.
Press Preview to read it the way a reader will.
Preview, and Create Translation, appear once the lesson has been saved with something in it.
Use Settings for the rest.
Daily Reading Plan sets the passages for this lesson, Templates puts a header subtemplate at the top or a footer at the bottom - keep it or undo it with Keep Changes or Undo - and Create Translation starts another language from what is here.
A banner saying you have view-only access means you may look and try things, but nothing you change is kept.
If the book is published, a saved lesson is what readers get. Do the work before publishing where you can.