In a regular @SpringBootApplication we would expect Spring Boot to scan the root package and all child packages below for components. An @AcrossApplication, however, encourages you to bundle your components in separate modules, and to only treat the application class as a descriptor for which modules should be added.

In fact, if you were to manually add a @ComponentScan directly on the DemoApplication class, starting the application would fail altogether (with a specific error message).

https://www.iodigital.com/en/history/foreach/building-a-modular-monolith-with-spring-boot-and-across