For Angular routing the routing option should have been enabled while starting new project.

1. To define a link you need to use routerLink instead of href followed by route in other side of equals like one in app.component.html

     <ul>
       <li><a routerLink="/">Home</a></li>
       <li><a routerLink="/aboutus">About Us</a></li>
       <li><a routerLink="/services">Services</a></li>
       <li><a routerLink="/contactus">Contact Us</a></li>
     </ul>
   

2. The list of routes needs to be defined in app-routing.module.ts. It is defined inside routes array list with path and component as key value pair like one below app-routing.module.ts

   const routes: Routes = [
     {path:'aboutus', component: AboutusComponent},
     {path:'services', component: ServicesComponent},
     {path:'contactus', component:ContactComponent}
   ];
   

3. The path and one defined assigned to routerLink in html should be same. The component is the name of the component class which represent individual page of application
4. If you are using absolute path then it would be /PATH which takes reference of component to be loaded form root url. If you are using relative path the it would be PATH (or) ../PATH (or) ../../PATH. Incase you are using relative path clicking the link of the same page from the page you are in should throw an error(Could not be verified)
5. Navigating to link could be acheived by two ways
   • Using RouterLink in a tags
   • Using router navigate method in ts code
6. routerLink always knows which route is currently loaded.router navigate method does not which route is presently loaded. So we always use activateRoute and pass relativeTo as parameter and pass relative URL as parameter

app-routing.module.ts

import { NgModule, Component } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';
import {AboutusComponent} from './aboutus/aboutus.component';
import {ServicesComponent} from './services/services.component';
import {ContactComponent} from './contact/contact.component';


const routes: Routes = [
  {path:'aboutus', component: AboutusComponent},
  {path:'services', component: ServicesComponent},
  {path:'contactus', component:ContactComponent}
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

app.component.html

<style>
ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  background-color: #333;
}

li {
  float: left;
}

li a {
  display: block;
  color: white;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
}

/* Change the link color to #111 (black) on hover */
li a:hover {
  background-color: #111;
}

.active {
  background-color: #4CAF50;
}

li {
  border-right: 1px solid #bbb;
}

li:last-child {
  border-right: none;
}
</style>
<div class="content" role="main">
  <ul>
    <li><a routerLink="/">Home</a></li>
    <li><a routerLink="/aboutus">About Us</a></li>
    <li><a routerLink="/services">Services</a></li>
    <li><a routerLink="/contactus">Contact Us</a></li>
  </ul>
</div>
<router-outlet></router-outlet>