ELI5: How do they make the HUD on a car appear to float above the front of the hood, and can they choose it's position?

There's no magic here, it's literally the distance the light travels. So trace an imaginary ray of light that goes from your eye to where you're seeing the digits/letters (let's call this point A). Where this ray intersects with the windshield will be our point B. And from point B, if you trace the ray as it reflects from the windshield (it'll go somewhere down, below the dashboard), at the distance equal to the distance between A and B, there's the actual screen. With just this one reflection it'd be too deep down to fit in the car, so this reflection process gets repeated a few times with mirrors to "fold" the light (similar to how it's done in binoculars with prisms)

So in short, the distance from the windshield to where the image appears, is the same distance the light reflected from the windshield traveled between the mirrors inside the projector.

Try shining and moving a flashlight while standing by a window, you will see its reflection seemingly appear on the other side at the same distance from the glass as where you're holding it.

Trying to guess the actual reason for your question - it'd probably be quite difficult to increase/decrease the "imaginary" distance significantly by modding it, as it would require relocating the components physically, or adding more mirrors/prisms in the light path.