What is a Application Sandwork in android and how it works?

Android applications run on a VM (Virtual Machine), and are completely isolated one from another due to the permissions Android gives each app. Basically each application on android is a separate user, and they have their own space on the "disk". That means Applications cannot access each other's space, Cannot uninstall or mess with each others data.

Only one user has access to OS and all applications, it's the root. So when people "root" their phone, they basically get root permissions, and can mess with every application including system services and libraries. But that's off topic