Backdrop has a Theme Debug mode that can be enabled and disabled via the a hidden setting. Theme debug mode can be used to see possible template suggestions and the locations of template files, as HTML comments in the markup of the page. You can enable the setting by placing the following config override in your settings.php file:
$config['system.core']['theme_debug'] = TRUE;
Alternatively, If you would prefer to use a button to enable/disable theme_debug mode, the contributed module Devel provides an option to enable the theme debug mode through the admin interface. Once you install and enable the module, navigate to Configuration -> Development -> Devel settings, enable the Enable theme debug option under the Core system devel settings section and save the configuration.
Once enabled, view the HTML source of any page generate by Backdrop. Theme Debug Mode will output additional HTML comments around all markup indicating what template or theme function generated that HTML. It will also include suggestions for how you can create more specific templates that might apply to a particular situation.
For example, anywhere a piece of content (a "node") is output, an HTML comment like this will be added:
<!-- FILE NAME SUGGESTIONS:
* node--1234--full.tpl.php
* node--1234.tpl.php
* node--blog--full.tpl.php
* node--blog.tpl.php
x node.tpl.php
-->The "x" indicates which template is in use, and any template with an asterisk indicates a more-specific template that can be used. The list of templates is from most-specific to least-specific. Having separate templates for different types of content is very common, as is having templates for different display modes (i.e. "teaser" versus "full"). Most sites with a custom theme will create at least one template for each type of content they create.