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.
Please refer the following document, to begin with, Site Layout.
Quick Steps to Enable Fluid Site Layout with Astra
Step 1: Make sure you have the Astra Pro Addon plugin installed and activated. How to Install Astra Pro Addon plugin?
Step 2: Activate the Site Layout module from the Astra Options under the WordPress Dashboard > Appearance > Astra Options
Step 3: Visit customizer [ Appearance > Customize > Global > Container > Site Layout ] and select Fluid as Site Layout settings.
Just as the name suggests — when fluid site layout is enabled, the entire website goes edge to edge of the browser, on all screen sizes.
If you have a website where you have a lot of content; for example blog, online magazine, or for any reason, if you need your entire website including header, footer, and main content area to span across the entire browser on all screen sizes, the Fluid Site Layout is for you.
The setting for Fluid Site Layout can be found under Appearance > Customize > Global > Container > Site Layout
Astra has provided an option to manage the padding on left and right.
On responsive devices, this padding is automatically adjusted.