Dans & à and being specific?
Specific location: Il mange dans le restaurant. Unspecific location: On vit à la campagne.
Why 'le restaurant' is specific but 'la campagne' is unspecific? If you think like English (how Duolingo explains it back there) both have 'the' and the listener probably knows which restaurant or county they talk about. Don't 'le/la' make something specific in French?
I don't think specific vs unspecific can be equated to definite (le/la) vs indefinite here. I would say (without being a fluent French speaker) the difference is that le restaurant is a more confined space than la campagne. If someone asks where you are, "at the restaurant" (which you have probably mentioned before because of the "the") is a more specific answer than "in the countryside" or "in the United States" which also use the definite article.
I'm French so I just know it like that but put into words, I would say that we use "On vit dans la campagne." less than the other example but it is still correct. "On vit dans la campagne" would mean that "We live in the countryside". "On vit à la campagne" means that "we live at the countryside."