What they did was kind of similar to how they did Luke Skywalker in The Mandalorian. They used the young actress as a stand-in who was then post processed via CGI into young Eleven. Her name is Martie Blair, and this is what she looked like at the Stanger Things premiere:There was some of both. Brown deaged was mostly in short, static scenes. Often, they just used regular Brown (with the kid showing up in mirrors). And sometimes they used the kid.
I suspect the kid wasn't much of an actor, because they didn't use her for many (maybe not any) of the speaking scenes.