Following is the definition. For following discussion assumes that the class com.example.app.TestApplication annotated with @SpringBootApplication.

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
  // ...
}

@EnableAutoConfiguration annotation

@EnableAutoConfiguration means to lookup org.springframework.boot.autoconfigure.EnableAutoConfiguration entry from \META-INF\spring.factories file of all jar files. Following is one of the entries.

# spring-boot-autoconfigure-2.6.11.jar!\META-INF\spring.factories
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
# ...

Specified AutoConfiguration classes coming from different files will be merged, imported to ApplicationContext and processed after finishing processing @Configuration classes. Spoken behaviour is controlled by org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.

@AutoConfigurationPackage annotation

@AutoConfigurationPackage is annotated in @EnableAutoConfiguration. This is for saving the path of base package (in this example com.example.app) and allow other configuration class (such as Spring Data) to scan annotation which is not handled by @ComponentScan. (In case of Spring Data, @Repository. Although it is annotated with @Component, but @Repository is annotated in interface that will not be processed by @ComponentScan.)

@SpringBootConfiguration annotation

Same behaviour as @Configuration, in addition that any properties defined will be merged into test class and will not be affected by the existence of @AutoConfigureXXX annotation.

@ComponentScan annotation

Scan component from base package (in this example com.example.app) and all childs. Exclude AutoConfiguration class found by @EnableAutoConfiguration (controlled by org.springframework.boot.autoconfigure.AutoConfigurationExcludeFilter) and child filter of TypeExcludeFilter (controlled by org.springframework.boot.context.TypeExcludeFilter).