Could you point how to create a custom plugin to prevent that our customizations are lost during updates?

Hi Experts

Could you point how to create a custom plugin to prevent that our customizations are lost during updates?

Directly customized theme functions.php


I followed these steps, but something else must to be done to get it working.

1. Create a folder and a file under plugins' folder to customize to avoid directly customize functions.php


2. The plugin is now avaliable

3. But if it's selected, it doesn't exists.

Thanks in advance!