Spring BOOT provides many abstraction layers to ease the development, underneath there are vital libraries which work for us. 

Below is the key function PERFORMING internally.

• Using @EnableAutoConfigure annotation the spring boot application configures the spring boot application automatically. 
• E.g. If you need MySQL DB in your project, but you haven’t configured any DATABASE connection, in that case, Spring boot auto configures as in memory database.
• The entry point of spring boot application is a class which contains @SpringBootApplication annotation and has the main method.
• Spring boot scan all the components included in the project by using @ComponentScan annotation.
• Let’s say we need the Spring and JPA for database connection, then we no need to add the individual dependency we can simply add the spring-boot-starter-data-jpa in the project.
• Spring boot follows the NAMING CONVENTION for dependency like spring-boot-starter. 

Considering above there are other internal functions which play a significant role in spring boot.