Xcode project inside project

They are called cross project references. From the link below:

Cross-project references are a convenient way to create relationships between two separate Xcode projects.

One method of creating shared frameworks is using sub-projects. You can also do it with single projects. More here:

https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Tasks/CreatingFrameworks.html

Other links:

Xcode 4 Adding a Sub Project

cross-project references aka project in project aka sub-project aka nested projects. This approach uses Explicit dependency^{[About] [How to setup]}

If you already have separate Xcode projects for your framework and application, you can take advantage of Xcode’s cross-project references to embed the framework in your application. Cross-project references are a convenient way to create relationships between two separate Xcode projects.

To define explicit dependencies between targets in separate project files, project files should be nested. This means that a target can set explicit dependencies of any other target within the same project file, or any nested child project file.

In XCode 4, the idea of workspaces^[About] is supposed to supersede the functionality of XCode 3s cross-project references