Blueprints (and code) are designed to be self-contained.
This is why I make so many plugins. Almost everything I submit later to “my module” which is a set of systems I designed like it was meant to be uploaded to the Marketplace and should work on any project…
When someone change base classes type/names they have to warn when submitting to Perforce. But in general, there’s no hard-references or strong dependency on anything outside one’s module.
Artists sync larger amount of assets, but the majority of my levels are sandboxes for tests that don’t even exist on the real project.
And yes, a new project is just a copy of the original uproject file and its Configs, as mentioned above that’s the same result of copying a project “preset”, the skeleton of the project is simply downloaded from P4. We just don’t use the same map assets and don’t commit maps either, somebody is responsible for keeping final versions of real maps on real project (level designer).