I can\'t find this information anywhere. Can someone explain how spring boot \'decides\' what the right scope is? Are the beans all singletons?
Spring Boot doesn't decide anything about the bean scope, this is plain Spring framework functionality. Default bean scope is singleton scope (meaning, one instance of that bean in the application).
Here is the official documentation:
https://docs.spring.io/spring-framework/docs/current/spring-framework-reference/core.html#beans-factory-scopes
Default scope for a Spring Bean in singleton.