Page templates
Page templates are used to change the look and feel of a specific page. You may decide that you want e.g. your homepage to look a specific way, that is quite different to other parts of your site.
You can find this window/option per page when you are in page admin editing mode.
Classic editor
Gutenberg
UX Builder
In UX Builder page edit mode press the cog wheel (in the side panel top right) to access the template options.