Information in the English language, or perhaps in most other Western languages as well, in relationship with the nature of tea is depressingly insufficient. A lot in popular circulation are more like propagation of myths. Sadly, many who have written them have not seriously studied the topic, some may have not even compare different tea plants.
The taxonomy of tea producing tea plants still needs much work, but you are right in saying that the two major varieties are v. sinensis and v. assamica. The origin of the latter is still in much debate, but most evidences point to the area in the northern part of Indochina and the southwest of Yunnan. The oldest surviving tea tree of the assamica variety is in Yunnan.
Beyond that, hundreds, if not thousands, of cultivars are in production. They can be different in appearance, growth pattern, resilience, resultant tea taste, etc. They can be the result of the hybrid between two other cultivars within the same variety, or cross. Some ancestry can be traced back, some still need work. A lot are carefully breed.
Da Hong Pao is one cultivar of the shuixian family of cultivars, officially speaking. I personally think that the shuixian family is likely to be a sub-variety, rather than simply a cultivar group.
I have many times mentioned the topic in the Tea Guardian. Just type in cultivar, or tea plant and you get many articles to read.
