Apple has the prescription lenses in the stores. If you bring your glasses, they have a device that will shine some lights through your glasses to determine your prescription and pop out the correct lenses needed for the Vision demo.

I wonder how do they manage that with all the corrections needed like for astigmatism? That's a huge range of combinations.

The intake form asks for certain things (like prism vision) and it also asks if you know your prescription ahead of time. I can't imagine that they'll have all of those specific fixes but, for the majority of people, you can simply go in and get the right prescriptions lenses. They even email you a link to get your specific prescription of lenses after the demo.