How to customised Shopify theme using code?

Shopify theme files contains HTML, CSS, and JavaScript for static content. But Shopify have different template language that is Liquid. This is basic script language for showing  dynamic content on the website.

Collections: The page which have "/collections/" shows all lists of collections. here in general language collections are categories of all products. which define to a particular group.

First page load is theme.liquid

the page responsible for catalog or we can say product listing page is: collection.liquid

Inside collection.liquid there is another template that is collection-template.liquid which is responsible for content showing in catalog page

For css we have file that is theme.scss.liquid which have css in scss pattern

For Contact us page there is predefined template will be here to use you have to select page.contact for this template the code of this template is inside the template that is page.contact.liquid so we can customize this page using this template.

you can add or delete new custom section by using settings_data.json file inside config folder which have schema name content_for_index.