Hi there,

Please try to read the document at: https://www.wpbeginner.com/beginners-guide/wordpress-child-theme-pros-cons/

and : https://developer.wordpress.org/themes/advanced-topics/child-themes/

#1: All css custom for theme you can add into file style.css
All function code php for theme, you can add into file function.php

When you use child theme and add code into it, if you update parent theme, your custom code wouldn’t lose.