How do I overwrite a page template? I created a bunch of products, but I need to style them per the designer's design. I know that I have to overwrite the template from /modules/contrib/commerce/modules/product/templates/commerce-product.html.twig but where do I put it in my theme?

Does it go to the root of my templates folder? What is the correct way to style the single product page?
To do this, just create a copy of commerce-product.html.twig from the product module in commerce to your theme's template directory. 

for more instruction to visit https://docs.drupalcommerce.org/commerce2/developer-guide/products/displaying-products/theme-product.