What happens internally during the Spring Boot application startup lifecycle?

SpringApplication.run() first prepares the Environment (property sources, active profiles), creates the appropriate ApplicationContext type, and invokes any registered ApplicationContextInitializers. It then calls context.refresh(), which loads all bean definitions, runs BeanFactoryPostProcessors, registers BeanPostProcessors, instantiates every singleton bean (triggering auto-configuration's conditional evaluation along the way), and publishes a ContextRefreshedEvent — finally, any ApplicationRunner/CommandLineRunner beans execute before the application is considered fully started.