it is a cornish rock cross you see up here most times. But it is far more than just throwing two breeds together. Both breeds are heavily selected, inbred and linebred for very specific characteristics. Then they are crossed, further bred up and that progeny is bred back to a linebred cornish again. At least that is how hatcheries here are doing it. They invest years in the technique and guard the exact procedures and genetic lines like gold.
All meat chickens come from 2-3 'breeders' in the US. They are all Cornish Crosses; with each company having their own proprietary genetics. If you look at their catalogs, they may offer 6-10 different 'Cornish Crosses' with different growth profiles, different FCR's and different environmental preferences. Everything you need to know about the commercial industry is in this ATTRA publication:
When Murray McMurray or anywhere else sells you "Jumbo Cornish Cross" they are getting their hatching eggs from one of the breeders listed in that publication, from a strain of birds with the largest growth size (the strain will have just some alphanumeric designation). They don't even know what goes into the breeding, they're just a clearing house. Actually, for the most part, very few of the online hatcheries do any breeding. They buy hatching eggs from more dedicated breeders (for better or worse).
Also, I would be IMMENSELY surprised if Tyson did any chicken raising, research or breeding themselves. The large producers (Foster Farms here in the NW) simply contract out to private farms for XXX number of broilers deliverable every YYY days for ZZZ dollars.
The average factory farm for these chickens earns about $0.10 to $0.15 per bird. That's not profit, that's the price Tyson will give them for it. If you google chicken factory farming, you will not enjoy reviewing the results.
Tyson (like Foster Farms) simply brings the birds into their processing facility, where they are procesesd, packaged and labeled then sold along wholesale to distributors.