The zip file is encrypted, that is why you or anyone else can see the content.
1. The purpose of the export feature, is to transfer assets between different accounts meaning import/export. For example, some organizations have 2 Subject7 installations prod/stage, and they want to transfer tests/artifacts between the two installations. Or for example for sharing amongst users from different accounts.
2. The format is encrypted ZIP.
3. You cannot read the content. Any particular reason why you want to?
4. Yes, you have to have permission and you can find the import on the root of the test library. However, on the same installation (instance of subject7, or account), it doesn't make sense. Please elaborate on your goal maybe we can help.