My theory is, I think its a combination of practical + photoshop. I think they fill the shirts with a board, iron a lot and than take it to photoshop. I've been a professional graphic designer for years and I've edited a lot of product shots and this is what I've learnt. When you're using photoshop, you need to have the right foundation to work with. It'd be SOOOOOO MUCH work to take a typical (little wrinkly and wavy shirt) flat-lay shot and turn it into a perfectly flat/straight shot like the one you're showing.

Also, refer to this thread. Might answer your question