Virtual machines (instances) are completely isolated from one another at the operating system level. Each instance has its own complete and independent Windows operating system. Data can only be shared between instances via network connections that can include access to shared storage.  

For example, the production instances in a Frame account have access to a shared network drive (P: drive). The P: drive is typically used as a common storage location for demo files.  By default, the P: drive is read-only when accessed from the production instances. We also have an option for a shared read/write drive (Team drive) that lets team members share data. So in these cases data moving between instances/users is a configurable option.