A good place to put interfaces and tools that aren't super expansive or accessed very often is the Utilities section in the backend. Adding new utilities is very simple, you just add a class that implements the Utility interface (you can extend the base class, then you only have to provide a title and the HTML body). Then register that utility class using the event
Utilities::EVENT_REGISTER_UTILITY_TYPES. See the documentation on Utility Types for details.
You can also add new top-level items to the navigation menu. First, add a controller that responds to a URL with the HTML content for your custom page. Then use the event
Cp::EVENT_REGISTER_CP_NAV_ITEMS to add a custom link to the control panel navigation that links to your controller action. See the documentation on Control Panel Sections for details.
Custom utility types, controllers and event hooks can be placed in a site-specific module. See the documentation on How to Build a Module. Those are usually placed in the
modules/ directory in your project.