How does BIRCH (Balanced Iterative Reduction Using hierarchical) CF Tree insertion algorithm work?
Since you mentioned you haven't read it yet, I suggest you start by reading the original paper (which was linked to from the Wikipedia page).  For research-level contributions, that's often a helpful place to start, if you're having trouble finding a place to learn more.

Wikipedia also links to some slides on BIRCH.  That might be worth reading first to get an overview for what they're trying to achieve, before reading the original paper -- but you'll still need to read the original paper.  The slides are not a substitute for the paper, especially if you want to know the specifics of the algorithm.

You should not expect a textbook or slides to give this level of details.  They might try to give you the general idea or the overall intuition, but if you want to know the full details, it's likely that you'll need to read the original paper.  It's not unusual to find that you need to go to the primary sources, for recent research like this.