Depends on the type of template you wish to change. Generally templates have a suffix to them. Some examples:

Page Suffix

These are used for templates on either individual pages or specific theme templates.

• page.php
• page-products.php
• page-cars.php

Single Suffix

Used for custom post types or just individual post types. The type of post type defined at the end is what the template will be used for.

• single.php
• single-books.php
• single-vans.php

There are others, and should you wish to learn more about these, you can take a look at the documentation provided by WordPress here:
https://developer.wordpress.org/themes/basics/template-files/

Dependant on the theme, the template files may be located in a folder named /templates or /template-parts or something along those lines.

On a side note: Since you mentioned you brought a custom theme, there might be options to customise the theme in the WordPress customiser, which can be found in the admin panel of your blog.

Hope this helps and Happy Theming!!