Prop traders at the major IB have access to every method/system known to mankind and huge pools of capital to trade with the very best in technology to boot.
However, very little trading is of the discretionary type that is found at the retail prop level. Most IB props are responsible for the oversight of the computerized/mechanical/black box trades that are generated by the models created by the Ph.D. geeks.
They also make money buying big blocks of stock from institutions--usually at a discount--and then reselling them to other customers at a profit. Goldman is the 800 pound gorilla in that business.