But why can't duolingo write this on the informations in the genitive theme? There is just one example of an -a , -я ending word мама-мамы. So we could just think that every word's genitive (ending with -a) form are made with -ы. Девочка девочкы. Why do they not write one sentence about that we have to use the 7 letter rule here too?
Yes so everyone confused with the whole у "something" есть, whatever has the object in that sentence is in gentive case, so for instance it's "у меня есть яблоко" not "у я есть", меня is in the genetive case because of possessions, it's quite easy to remember the format as the litteral meaning of this is "by me there is... " hope this helps!