Controllers bundle actions that handle business logic for each route within Strapi’s MVC pattern. This documentation demonstrates generating controllers, extending core ones with createCoreController, and delegating heavy logic to services.
Middlewares alter the request or response flow at application or API levels. This documentation distinguishes global versus route middlewares and illustrates custom implementations with generation patterns.
Strapi policies are functions that execute specific logic on each request before it reaches the controller. Policies can be customized according to your needs.
Strapi routes handle requests to your content and are auto-generated for your content-types. Routes can be customized according to your needs.
Strapi's Server API for plugins allows a Strapi plugin to customize the back end part (i.e. the server) of your application.