There are some solutions, make sure to check them all :)
Option1:
The router outlet will emit the activate event any time a new component is being instantiated, so we could use (activate) to scroll (for example) to the top:
app.component.html
<router-outlet (activate)="onActivate($event)"></router-outlet>
app.component.ts
onActivate(event) {
   // window.scroll(0,0);
   window.scroll({ 
           top: 0, 
           left: 0, 
           behavior: 'smooth' 
    });
    //or document.body.scrollTop = 0;
    //or document.querySelector('body').scrollTo(0,0)
    ...
}
As the smooth scroll is not implemented well in Safari, use, for exemple, this solution for a smooth scroll:
onActivate(event) {
    let scrollToTop = window.setInterval(() => {
        let pos = window.pageYOffset;
        if (pos > 0) {
            window.scrollTo(0, pos - 20); // how far to scroll on each step
        } else {
            window.clearInterval(scrollToTop);
        }
    }, 16);
}
If you wish to be selective, say not every component should trigger the scrolling, you can check it in an if statement like this:
onActivate(e) {
    if (e.constructor.name)==="login"{ // for example
            window.scroll(0,0);
    }
}
Option2:
Since Angular6.1, we can also use  { scrollPositionRestoration: 'enabled' } on eagerly loaded modules and it will be applied to all routes:
RouterModule.forRoot(appRoutes, { scrollPositionRestoration: 'enabled' })
It will also do the smooth scrolling, already. However this has the inconvenient for doing it on every routing.
Option3:
An other solution is to do the top scrolling on router animation. Add this in every transition where you want to scroll to the top:
query(':enter, :leave', style({ position: 'fixed' }), { optional: true })