What does 'auto-configuration' actually do under the hood in Spring Boot?

At startup, Spring Boot scans `META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports` for a list of `@Configuration` classes, each guarded by conditional annotations like `@ConditionalOnClass` or `@ConditionalOnMissingBean`. It only activates a configuration if its trigger condition is true (e.g. an embedded Tomcat bean is only created if `spring-boot-starter-web` is on the classpath and you haven't already defined your own server bean), which is why adding a starter dependency is often the only setup step needed.