How to create a "Theme Panel" in the admin for a template?

Question

PHP

In more expensive templates in the WordPress admin panel , it is possible to change the settings of the theme itself, set by the developer, i.e. theme panel.
I want to know how it is done, maybe there are some articles on this topic and how to work with the insides themselves (changing the logo, choosing the main color of the theme, etc.) so that the person who buys my template does not mess around in the code , and through this panel made the main changes.
I hope for a proper answer.

Answer 1

How are fully customizable themes created?
Is there a plugin for creating custom options of any type with their grouping?

Answer 2

https://codex.wordpress.org/Theme_Customization_API