The Child theme is for theme customization, you can add the some changed css/js/php codes in to child theme folder, the changed files will not be covered when theme updated(because the files in air-theme folder will be covered, the files in air-child folder will NOT be covered).
2) Install the child theme, it is same as installing parent theme, the installable file is “air-child.zip”. Activate child theme after installed.
4) If you want to change some PHP files: like header.php, footer.php or php files in template folder, please clone the PHP files, put the cloned PHP files into child theme folder. Please make sure the folder tree is same as parent theme. Like:
You can change the cloned files into child theme folder.