I have a guess that may explain all the cases: By default, change detection is triggered from the root component and traverses all child components. Unless a component calls the detach() method to detach itself, change detection will not continue to traverse it and its child components. In the case of manually triggering change detection (by injecting ChangeDetectorRef into the component and calling detectChanges()), change detection will start from the current component and traverse from there.

While traversing each component, for components using the default change detection strategy, it will directly trigger a UI update. For components using the OnPush change detection strategy, it will check whether the component has been marked for UI update, which we can call the 'flag' (the component will be marked as flag = true when one of the following conditions is met: a. event triggers; b. input property changes; c. Observable bound via async pipe calls the next method). If the condition is satisfied, the UI update will be triggered. After the UI update is complete, the flag will be set to false.