When I change something on a page or a product page, other pages change too. How can I get rid of it?
In Shopify, by default, all of the pages use the same page template. Similarly, all of the product pages use the same product page template. That being said, except for the individual data (title, description...), all the changes you make to a page or a product page (via the theme settings panel, including adding sections or blocks) will be considered representative, and it will apply to all pages or product pages.
So, if you want to separate the settings for each page, you need to create a page template for each.
Let's assume that I have a page called "Drinking Culture" and I want to create a separate template for it based on Default page template. Here's what I should do:
Then, in the page in admin, I will assign the template "drinking-culture" for it, like this:
By doing this, now the setting I made for Drinking Culture should be unique and won't impact other pages.
For product pages, you can also use this method.