How to make changes in the website created using theme without making changes in theme file by creating new extension

We want to make changes in the website but the changes shouldnt be made in the theme files than we need to create extension and in that extension we need to make changes

1. first, we need to create extension, then we need to create template file in in extension.
2. than we need to copy template file from the theme and paste that to template file in the extension
3. than make the changes in the extension tpl file