How does RxJS create Observable objects based on events?

RxJS provides some functions for creating Observable objects. We can create Observable objects based on events, timers, Promise, AJAX, etc.
Below are a few examples of creating Observable objects based on events.

this.outsideClick$ = fromEvent(document, 'click').subscribe((event: MouseEvent) => {
    
    
            this.onClick(event);
        });

fromEvent(window, 'resize').subscribe((e: any) => {
    
    
                this.windowSize = e.target.innerWidth;
            }),

fromEvent(document, 'mousedown').subscribe((event) => {
    
    
                    if (!event['isOpen']) {
    
    
                        this.close(null);
                    }
                }),

 fromEvent(window, 'mousewheel').pipe(
                            debounceTime(250)
                        ).subscribe((e: any) => {
    
    
                            // handle e
                            }
                        }),