Where are the actual lessons?
Where are the actual lessons? Some sections have lessons that can be reviewed before taking the quiz, but most so far just launch right into quizzes -- for material that is completely new!
What I've done in these cases is when I find a question with words or phrases or conjugations & such that are new: I just type gibberish into the answer just to see what the right answer is, and then memorize it for the next time it comes around. But this does little in the way of instruction. Why do some sections not have lessons attached?
Duo lessons consist on taking little exercises and learning from/during/with them: it's Duolingo methodology.
The Tips and Notes (I guess that's what you called "lessons") are just a plus that is sometimes added by contributors to the course but isn't per se part of the teaching in Duo's methodology.
By the way, most of users (=apps users) don't have access to them and according to staff most of users having access to them don't read them at all.