What process or thread is responsible for handling the user interface in the system?

In iOS, the main thread is responsible for handling the user interface. All operations related to updating the UI, processing user events, and interacting with UIKit are executed on this thread.

If long or blocking operations are performed on the main thread, the interface will 'freeze' or become unresponsive. Therefore, heavy tasks are usually offloaded to background threads, while UI updates are always done on the main thread.

Example of updating the UI on the main thread:

DispatchQueue.main.async {
    // update interface elements
    self.label.text = "Updated"
}