They are in the same repository. For example with Maven if you run any task from the sub-project which depends on adjacent sub-project, maven will look for it

They are in the same repository. For example with Maven if you run any task from the sub-project which depends on adjacent sub-project, maven will look for it in ~/.m2/repository. So you have to mvn install every time you're doing any change in any sub-project which is PITA when you're changing many sub-projects simultaneously (and if you did not mvn install, you'll have old version in repository and new sources in IDE and things go wrong way in a completely non-obvious manner). But if you're running maven commands from root pom, sub-projects will properly reference each other. So the proper way is to run all commands from root repository and specify -pl. But then you'll run into other issues, as not all plugins will work correctly this way (for example mvn exec:java). With gradle it just works. You're running command for any sub-project and gradle will properly find any dependant sub-projects, build them, put them into classpath, etc.