In the following example you can see how to change page layout to left.

Best solution is to add custom actions and filters in Child Theme functions.php file. But also you can create your own plugin and use this codes in your plugin.

In the below screenshot, you can see how to find hide header page meta key. First edit your page in backend then find your option and then open your browser inspect element feature to find the input name or data-depent-id attribute. Exactly like below shot you can find key. For hide header key is hide_header