Lots of academic reading, very philosophical and you need to construct your own opinions of questions surrounding history such as what the purpose of history is or if history can be objective. It's about historiography, not history itself.
just my personal opinion
Q1: What exactly do you learn in history extension?
We don't actually learn a lot, if any, history in history ext. What we do learn is the history of history and this is called historiography.
Q2: Is it hard?
I thought it was pretty hard because it's a completely new way...