I know what a hole saw is. The thing is you aren't going to be cutting through hardened steel with a normal hole saw, assuming the cash box is metal. They could probably have made the enclosure for the cash 8 inches taller and put the computer inside of that, and routed the cables for the modem, the pin pad and the screen through

Making a hole in hardened steel is not easy. If you use a hole saw, it will either need to have diamond abrasive, or you will need carbide bits. If you don't use a hole saw, you are going to be using an angle grinder or a plasma torch.

Getting in fast will either generate a ton of heat or a ton of noise, or both. Plus, carbide and diamond tools are super expensive. Angle grinders, not as much, but I'd notice someone with a 3 foot trail of sparks behind them before I'd notice someone running a drill at a few hundred rpm.

Anyways, I do think they probably used a hole saw, which leads me to believe the computer was located behind some combination of aluminum/plastic/ordinary steel. Which is pretty ridiculous