Have iOS apps. Can I use them on my Mac Mini?

At the moment, you can't really do this. Although I suspect that Apple is looking to integrate iOS and macOS in the future so that this sort of thing can be done (purely speculation).

GarageBand on the Mac is different from the iPad version (it's more comprehensive) and it has been available on macOS for far longer than on iOS.

It is possible to run iOS apps in the Simulator app that comes with Xcode. However, it's not as simple as downloading an app from the App Store. You'd have to get the source code for the app, compile it in Xcode and run in the Simulator. Most apps being closed source, this won’t be possible!

In general no - one reason is that the processor in the iPads/iPhones etc is an ARM one and in Macs is now an Intel x86 and used to be a PPC, the instruction sets are just completely different.