You can choose to interconnect them but that is neither required nor default. It depends on what you want to build. You can have a simple policy on a resource

You can choose to interconnect them but that is neither required nor default. It depends on what you want to build. You can have a simple policy on a resource that just allows something, or you can make a complicated bi-directional set of policies that refer to specific principals, resources and actions to constrain it more. That will never go away if that is what you need to build. And it is still optional because if you don't want to build that, then you just end up with your single, not-inter-connected policy.