The specs (there are multiple) are enormous and you honestly have zero hope of fully implementing one from scratch. It's thousands and thousands of pages covering everything from the physical layer all the way up to deep application-layer integration. Manufacturers rely on support libraries and hardware that does most of the work, and until only the last few years most of those on the whole have been total garbage (again because the specs are so massive it just costs too much time and effort to implement them). I don't expect anything less than a team of 10 people all with a mix of senior hardware and software/firmware experience could fully implement bluetooth & bluetooth LE from scratch in less than a year.