Android Studio: Submodules in Project Structure Hierarchy

Question

I am building a multi-module app, and I have included the modules in the project-level settngs.gradle file, in this manner:

include(":sub_module_1")
project(":sub_module_1").projectDir = file("../path/to/sub_module_1_folder")

include(":sub_module_2")
project(":sub_module_2").projectDir = file("../path/to/sub_module_2_folder")

include(":sub_module_3")
project(":sub_module_3").projectDir = file("../path/to/sub_module_3_folder")

Then I implement them in the app-level build.gradle file, in this manner:

implementation project(":sub_module_1")
implementation project(":sub_module_2")
implementation project(":sub_module_3")

So far, so good. The project builds and runs as expected.

However, in the Project Explorer (both Android & Project tabs), some modules appear inside the main project, while some appear outside of it. But all of the sub-module file folders are located outside of the main project folder. Here's an illustration of the Project Explorer (Android):

> sub_module_1
  |__ manifests
  |__ java
  |__ res
> main_project
  > app
    |__ manifests
    |__ java
    |__ res
  > sub_module_2
    |__ java
  > sub_module_3
    |__ manifests
    |__ java
    |__ res
> Gradle Scripts

Q: What could be causing this?