It's not a breed. It is a hybrid. Many of them come from commercial hatcheries or from parent stock from commercial strains. Many are not "breeds" as parent stock either, but proprietary "red" roosters are used over proprietary "white/silver" hens. They've been bred for the hen laying houses for decades. The parent stock is no longer really a recognizable breed anymore, if that makes sense. They've really been selected, tested and tweaked for top laying, good feed conversion and so forth.
Of course, local hatcheries can "roll their own" sex links as well, using whatever they have available in the breeding stock that will work.
Bottom line is that in spite of the marketing PR, we will often never really know. It's pretty much trade secrets.
The sex link thing is a one trick pony. In the subsequent generations, they won't breed true. If you breed a red sexlink to red sexlink you get all kinds of interesting things.