TBB provides some of its functionality in the form of templates, but it is not just a template library. It is specifically a dynamically linked library to allow its applications to be brought together into one program without explicit coordination.
Well, you have access to the ultimate authority. Intel Threading Building Blocks is an open source project and you can find details as deep as you want to go by looking at it. Code reading is a great exercise for learning structure, and as someone who supports this library, I'm constantly visiting newer versions of the source as they appear to understand how that code is evolving and structurally changing as new features appear.
Because of the noted distinction between the template header files and the separate library source, I find navigation easiest done by having some tool to link function calls to definitions so that you can functionally navigate across file boundaries. My personal preference is the tags feature in vim, combined with a robust ctags. From the header file directory I specify:
[plain]ctags -R ./* ../../src/*[/plain]
This way my single tags file links function references among and within the header files but spans to all of the source, including test files, tbb and tbb_malloc. But that's pretty arcane. I'm sure there are other navigation tools that would suffice.