How And Where To Use Custom Sidebars?

You can use your custom sidebar where sidebar selection options are available such as page layout, post layout, product layout, header, mini header, footer sections, etc.

to use sidebars in page/post/product layouts, refer this article. See below image for more understanding.

How to use sidebars in section builder (Header, Footer, Mini Header)

You can use custom added sidebar with widgetised sidebar shortcode where you can add sidebars in header, footer, etc. as per your requirements.

You can also select custom added sidebar in widgetised sidebar shortcode.

For more details, please check below image.