[Jerry Smith]"If I import an AI or a PS file into AE and then change the AI or PS file those changes show up in AE. Something like this definitely happens. I've seen it! :)"
Importing footage creates a pointer in the After Effects project to the media. As you note, changes to the media will be reflected in Ae.
[Jerry Smith]"On the other hand, if I import AE project 1 into AE project 2, close AE, then open AE project 1 and make a change, and then close it and open AE project 2, nothing will have changed in AE project 2."
Importing a project is different; Ae does not point back to the original AEP. Rather, it copies the contents as they were at the time of import into the new AEP.