In fact, the ownership is not important, the most important is the ability to control. Even if you have no money at your personal account, but you can control a bank to drive large acquisition operation, you can achieve much more influence than Bill Gates. And I suppose that banks can freeze Bill Gates' account easily, so Bill Gates really have no control over any bank, he is the game player while the banks are game designer
Before, nobles and kings used to have control over banks, but Rothschild family invented the way to virtualize their wealth into stocks/bonds/asset certificates around the world, thus avoided control from the greedy monarchs
Bill Gates's maternal grandfather was J.W Maxwell, the creator and president of Seattle's National City bank. At a guess he probably owned a large share of that bank, and controlled it. Bill Gates's family was probably already extremely rich before he was even born. Perhaps the money he has added to that of his extended family would already make a trillion.
https://en.wikipedia.org/?title=Bill_Gates