Default Bootloader Source

It is not, but you could disassemble it if you really wanted too. Created my own annotated listings here.

There are many better examples you could draw from. ST also has a handful of IAP examples using assorted interfaces and methods.

I would generally suggest creating something that can work with standard tools (ie a terminal app) rather than something that needs a custom app or drivers on each of the host operating systems you will be expected to support.