Courses do not teach us how to add details into a sentence!
As you may have noticed, for example in a German course, it doesn't really teach the users how to add detail into the original sentence, for example they only teach us "They have a shirt", but do not teach us how to add "They have a clean white shirt". I am not saying that Duolingo isn't efficient, honestly it teaches me languages better than my actual teachers, Duolingo is the most amazing website I ever used, but it could always improve to be even better!
I agree with you, i need to use youtube and other materials for learning, because when you learn a new language you need to learn WHY is the sentence like that, or why the words are in that order... duolingo is really great and really helps but i agree it should improve!
Is that your experience with German? It may improve as you go along. I mean, not that I've done much German, but with Spanish there are whole lessons dedicated to adjectives like the "details" you're mentioning, so naturally these lessons will include sentences that utilize one or more of them, so you see how it's done. I actually prefer language learning where you aren't handed a book of rules on a silver platter. It's easier for me to do lessons and pick up beginner's eBooks on Amazon and figure out that Spanish never uses an Oxford comma, than to be given that along with 20 other rules to memorize, if you get what I mean :)
Well in the Polish course they teach you that later so I guess it is the same for German but if you feel that Duolingo is not enough, you can still practice your vocab on Memrise, learn more phrases on Clozemaster, read online newspapers, write little text or story and of course listen to some audio records and practice with natives.
I agree with the others who say that you haven´t progressed far enough in the courses to have been exposed to much complexity yet. It will come.
You will probably feel the need to incorporate even more "detail" when you finish the courses, but by then you will have a good basis in your languages and can go on to use other resources (if you haven't already).
There may be a very practical reason for the (minor) delay in teaching adjectives. It's been quite a while since I took German, but I believe endings of adjectives change with gender, case, and possibly number. That's a lot to throw at a relative newbie.