I have not studied it in detail. I think the basic idea is that you some neanderthal DNA seguences, some denisovan DNA seguences and modern human DNA from all over the world. Europeans have some of the neanderthal, east asians have some of the denisovan and africans have neither. Homo Sapiens Sapiens evolved in africa, some stayed and became africans, some left, entered europe and bred with neanderthals and became europeans and some entered east asia, bred with denisovans and became east asians.
Besides the Neandertal and Denisovan there is a third ancient hominin that interbred with Homo Sapiens.
Sub-Saharan Homo Sapiens has some genes from a sub-Sahara hominin called the Ghost species.
see also: http://www.buffalo.edu/news/releases/2017/07/028.html