Short video on how polyurethane foam is made. It is bought in liquid form in two chemical components, that once mixed it reacts and the foam is created. There are rigid and flexible foams as well as different densities, check which one you are buying!

I am surprised and grateful for so many visits and questions, I am terribly sorry I cannot reply to all of you.
I am not an expert on foams or chemist. I am an industrial designer, the material I used in this demonstration is this one:
https://www.smoothon.com/products/fo...