How does the app do it, modify the page layout of the selected product?

[Old_driver]

I want to do app development and modify the selected product page. Does anyone know how I can modify the selected product page?

[w3trends]

Was there suppose to be a link to a page?

You can use a page builder app to design a new Product page and then use that custom template instead of the basic product template:

https://apps.shopify.com/search?q=page+builder&st_source=autocomplete