This will set the default site layout, but it can be overridden at page level when editing a page or a post. But just so you won’t have to override every time for each post, we split the option into 2 different options. So you can set the site layout for general and for posts.
- From the WordPress admin, go to Appearance > Customize.
- From the Customizer left menu, go to Typer > Layout.
- Choose one, two or no sidebars.
The same settings are available for posts.