"Update Existing" resources etc - what exactly is being updated? How come all my projects bleed together for resources?

Hmm.  'Bleed together for resources' - this sounds a bit like a query about all projects sharing the same pool of resources. Which is correct, and almost the whole point of an enterprise-wide project management system. You can partition the resource dictionary such that you can emulate project-specific resources.

If, on import, you chose 'use existing' then the project might not match the original exported project, calendar details, resource availability etc.