Project Hierarchy: Create subdesign from Quartus project

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Hello everyone,

This might be a very dumb question with a very obvious answer, but I couldn't for the life of me find an answer on Google.
In Intel Quartus, if I wanted to create a "sub-project" from an existing project, so I can integrate it in a new one, say as a functional block in the block design file, how would one go about it?
There has to be project hierarchy because it says so in the UI, but I can't find out how that works.

Thanks