"La nourriture des jeunes éléphants"
That is strange to me. I don't know why that is. I would read that sentence as "He has of big dogs" which is obviously wrong. I would have said "il a des grosses chiennes" which would translate to "he has some big dogs" or "he has big dogs". I don't know about that. Maybe I'm not aware of a language/grammar rule. If I find out, I'll let you know. Sorry I can't help.
Check the book : Grammaire progressive du francais by Maia Grégoire and Odile Thiévenaz. It said : En géneral, devant un adjectif, "des" devient de, comme "de bons amis" ou "de beaux livres". So I believe there is a mistake here. The right sentence should be "La nourriture de jeunes éléphants"
However, this kind of grammar can be complicated for learners at the beginning, that's why Doulingo put "des" so that it's simpler. Later, they may introduce it.
Des means of the.
Of the means some but not all. I eat of the bread means not a particular bread, not all the bread but just of it. Therefore des means some.
Of the also means belong to. The food of the elephants means the food belonging to the elephants.
When des is used to indicate possession it is not contracted to de.