I can only speak for how IESE does it (or rather did it a few years ago, not sure if that has changed in any way).
Generally, for each class you have on a day, you have one case. You read the case at home, do the numbers for it (most cases do have numeric data that you do some calculations with), discuss it with a few other students before class in your learning team, and then the case is discussed in class.
As for fundamentals/theory, there's a few ways to get that. Depending on the class, you might be assigned book chapters to read, or you will receive teaching notes (which are basically also just like book chapters) to read that cover the fundamentals you need for the next case(s). Also, depending on the subject, being case based does not mean there will be zero lectures. Especially in classes like finance, there will be days that are just lectures by the professor or at least a part of the period is a lecture.
You should check out HBS' homepage - If I remember correctly they have videos on there explaining the case method.