For a good introduction on how the layout system works in Backdrop CMS, see the following pages in the user manual:
https://backdropcms.org/user-guide/layouts-and-templates
You will find Layouts under the the Structure menu in the Administration Menu.
Note, Layouts are new to Backdrop and are not a part of Drupal 7. You will not find any Drupal 7 resources that help you with the Backdrop CMS Layouts.
(More content coming...)
