This is a premium feature available with Astra Pro Addon plugin. To use these Pro features, you need to have the Astra theme along with the Astra Pro Addon installed on your website.
You can activate the module from Appearance > Astra Options > Site Layouts.
With Site Layout module, you can manage the overall layout of the website.
Once you activate the module you can find Site Layout setting under Appearance > Customize > Global > Container > Site Layout
The layout chosen will be applied to the outer wrapper of the site. The default layout will be “Full Width”. Options available with Site Layout modules are explained in individual articles linked below.