Mizuu downloads various content to your device, so it’s always available for offline use.
The download folder is called data/com.miz.mizuu/ on your default storage device – your internal memory storage or external memory (SD card).
This folder is not deleted when uninstalling the application, so you have to do that yourself.
The reason for this is that I want to provide full transparency, and show users exactly what the application is storing on their device. If the data were to be deleted when uninstalling the application, it would not be possible for users to view the downloaded content.