The same way humans use tanks? Except for maybe the buttons and levers being a little bigger.

magic?

You see they use the power of magic from the unicorns for an engine, I like to imagine that in the tank they just have a unicorn that acts as the engine and moves it forward as they feed it food

More like bigger buttons and levers, that and on the wiki one of their semi-good enough explainations is that their hooves are like magnets and they can use em to stick things to em