What does the @SpringBootApplication annotation actually do?

@SpringBootApplication is a single, convenient meta-annotation that bundles together three separate annotations, each contributing a distinct piece of behavior:

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication { ... }

1. @SpringBootConfiguration — itself a specialization of the core Spring @Configuration annotation, marking this class as a source of bean definitions (so @Bean methods inside it are honored) and identifying it, specifically, as the application's primary configuration class (useful for testing tools to locate it).

2. @EnableAutoConfiguration — the annotation that actually triggers Spring Boot's auto-configuration machinery: based on what's present on the classpath and what beans already exist, Spring Boot conditionally activates a large set of pre-built configuration classes (for a DataSource, a DispatcherServlet, Jackson's ObjectMapper, and so on).

3. @ComponentScan — scans the package containing the annotated class, and all its subpackages, for @Component/@Service/@Repository/@Controller-annotated classes to register as beans.

That third point is exactly why the conventional Spring Boot project structure places the @SpringBootApplication-annotated main class in the application's root package — anything outside that package tree (and its subpackages) won't be picked up by component scanning unless explicitly configured otherwise.

com.example.myapp
├── MyApp.java              @SpringBootApplication — scans everything under com.example.myapp
├── controller/OrderController.java
├── service/OrderService.java
└── repository/OrderRepository.java

You can use the three annotations individually instead of @SpringBootApplication if you need finer-grained control (e.g., customizing @ComponentScan's base packages independently) — @SpringBootApplication is purely a convenience default for the common case.