Magic Mouse with iOS 12.X?

The Magic Mouse does work with iOS 13, you'll need to use a PIN code of 0000 to pair it. If you have trouble getting the mouse into pairing mode, remove the batteries from the mouse and re-insert them after a short while. This forces it into discovery mode, where you can pair it.

The Magic Mouse 2 is slightly more complicated, as it did not work in the betas of iOS 13 at all. It is however supported in the final release of iOS 13 - however note that the iOS 13 support for Magic Mouse 2 so far includes only pointing support and clicks. There's no support for any gestures.

There's no way to enable the Magic Mouse on iOS 12.

The video you have linked to shows an iOS Simulator running on a Mac. Here the mouse is obviously supported. This doesn't make it possible to get it working with a real iPad with iOS 12.

A few apps exists with mouse support for iOS 12 and earlier. They do not support the Magic Mouse though. They only support special hardware, and it's not supported system wide - but only limited to a single app. For example you can get a Swiftpoint GT Mouse for use with the VMware Horizon Client for remote desktopping.