https://youtu.be/yJQbO-NSbx4?t=169

The electrical box sits inside the cabinet. It requires a 40 amp breaker with #8 AWG wire since it can consume up to 7,800 W at 240 VAC. This means while it connects to the standard 120 V electrical supply is has two hot wires (red and black) rather than 1 (black) to achieve the 2x voltage. The double oven will typically come with flexible conduit but you must provide the rest (aka electrical box, cover, etc).

https://youtu.be/yJQbO-NSbx4?t=351

If your cabinet is too wide you need to install side wood pieces to fill in the gap and give the double oven frame something to screw in to. Screw the extra side pieces in on either side of double oven cabinet so they don't show and then screw the frame of the oven to those. You'll also probably want to add some fancy trim on the edges to cover the gap/extra piece of wood you added between the oven frame and the side of the cabinet.

How to install a double wall oven.