What is the type of scroll event in angular

It would be Event as a type.

onScroll(event: Event) {
   const value= event.target;
  }

Fired at the Document or element when the viewport or element is scrolled, respectively. https://drafts.csswg.org/cssom-view/#event-summary

An event can be triggered by the user action e.g. clicking the mouse button or tapping keyboard, or generated by APIs to represent the progress of an asynchronous task. https://developer.mozilla.org/en-US/docs/Web/API/Event

The easiest way to find out (not using the docs) would be to print the input:

 onScroll(event: any): void {
    console.log(event);
  }