It's incredible how this comment got a bunch of replies not understanding the question.
Explanation: Tether is not holding a stack of paper bonds in a safe. They must use some broker (or similar) to operate and hold the treasuries for them. And they are hiding how this is working. So the parent's question is who do they (Tether) hold it (treasuries) through (as in what broker/investment bank/whatever).
My answer is nobody outside Tether knows. And they are one of the dodgiest companies out there. We are totally audited, bro, trust me. They could be doing leveraged investments. Or they could just be Madoff 2.0. Nobody really knows. And the US government seems to be looking the other way because they are desperate for any buyers of treasuries.