If you’re preparing for a French test, you should focus mostly on the second category but if you’re aiming for a C1 or C2 level you should also get some exposure with more formal language.
Best resources for formal language
Read classic French litterature and some of the contemporary writers (André Malraux, Antoine de Saint-Exupéry, Albert Camus, Georges Simenon, Amélie Nothomb, Jean-Christophe Ruffin, Delphine de Vigan…).
Best resources for standard to casual language
Read blogs about topics you like and some contemporary novels (some books by Albert Camus have simpler language, Anna Gavalda, Fred Vargas, Leïla Slimani, Gaël Faye, Guillaume Musso, Marc Lévy, Maxime Chattham, Michel Bussi…).
Listen to podcasts, TedTalks and some Youtubers (in their mid-thirties, for example Norman, Natoo and Paul Taylor, or over if possible because there’s clearly been a shift in the language in the last decade).
Watch movies and series on Netflix but select the ones adressed to kids or to an older audience (35 and over). One of my favourite series in that category is “Call my agent” (“10%-Dix pourcents” in French) because it’s casual workplace French, the dialogues are very realistic and actors are fantastic.
Best resources for very casual language and recent slang
I’m not an expert in that one (not my generation!) but here are a few suggestions to start with.
French rap and RnB
Streetfrench.org for explanations about the slang
Youtube channels made by people in their twenties or younger
French reality TV