Hello.
It’s a reusable section, but is the theme such as Debut?
If so, the insertion into the product page etc. is a static rendering using the {{section’○○○’}} (○○○ .liquid section file) tag inside the product.liquid file. It seems that.
In the case of static rendering, changing the section settings seems to rewrite everything to the same content as you said. Below is a description of the official documentation.
You can include a statically-rendered section in multiple theme files. However, only one instance of the section exists. If you change section settings in one location, then the change will be applied to all locations where the section is rendered.
( https://shopify.dev/themes/architecture/sections )
With dynamic rendering (sections can be freely swapped on the theme customizer), multiple settings can be made for the same type of section, but with a theme such as Debut, it is dynamic only on the homepage (index). It is a specification that can not be rendered properly.
However, on Shopify’s new specs and new theme “Dawn” last month, you can now dynamically place sections on all pages, including product pages.
(You can create as many product page templates as you like for each product.)
So I think it’s a good idea to try the new theme Dawn.
https://www.shopify.jp/blog/unite-2021
Currently, the Dawn theme is only in the development store, but I’ve heard that it will soon be available in the free theme store on the merchant’s page.