They usually make their own proprietary softwares. Dont' forget that they have some deep pockets and they also have to handle customer orders. The customer orders vary depending on if the B/D is a wirehouse to a wholesaler to a blocktrader. Their software is a little different then most of the daytrading platforms seen on the street. They manage orders. Their analytics are most likely different systems such as Rueters or Bloomberg. Order entry and positions are probably on a separte system. Combining the two slows down the computer. But other than that most of the software is custom designed to the B/D's needs.