First, I’m assuming you mean how MongoDB uses B-Trees for their indexes because there is no “B-Tree” index I just wanted to clarify.
But I don’t think there is any documentation on how it is implemented, but there are some older posts that talk about it and link some pages that might be helpful to start to understand. It seems like if you understand the core concepts of B-Trees then you can get an idea of how they are used for indexes.