Edit

Demonstrates overlays.

The popups are created using Popovers from Bootstrap.

main.js
import 'ol/ol.css';
import Map from 'ol/Map';
import OSM from 'ol/source/OSM';
import Overlay from 'ol/Overlay';
import TileLayer from 'ol/layer/Tile';
import View from 'ol/View';
import {fromLonLat, toLonLat} from 'ol/proj';
import {toStringHDMS} from 'ol/coordinate';

const layer = new TileLayer({
  source: new OSM(),
});

const map = new Map({
  layers: [layer],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

const pos = fromLonLat([16.3725, 48.208889]);

// Popup showing the position the user clicked
const popup = new Overlay({
  element: document.getElementById('popup'),
});
map.addOverlay(popup);

// Vienna marker
const marker = new Overlay({
  position: pos,
  positioning: 'center-center',
  element: document.getElementById('marker'),
  stopEvent: false,
});
map.addOverlay(marker);

// Vienna label
const vienna = new Overlay({
  position: pos,
  element: document.getElementById('vienna'),
});
map.addOverlay(vienna);

map.on('click', function (evt) {
  const element = popup.getElement();
  const coordinate = evt.coordinate;
  const hdms = toStringHDMS(toLonLat(coordinate));

  $(element).popover('dispose');
  popup.setPosition(coordinate);
  $(element).popover({
    container: element,
    placement: 'top',
    animation: false,
    html: true,
    content: '<p>The location you clicked was:</p><code>' + hdms + '</code>',
  });
  $(element).popover('show');
});
index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Overlay</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep"></script>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,URL,TextDecoder,Number.isInteger"></script>
    <script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css">
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.0/js/bootstrap.bundle.min.js"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
      #marker {
        width: 20px;
        height: 20px;
        border: 1px solid #088;
        border-radius: 10px;
        background-color: #0FF;
        opacity: 0.5;
      }
      #vienna {
        text-decoration: none;
        color: white;
        font-size: 11pt;
        font-weight: bold;
        text-shadow: black 0.1em 0.1em 0.2em;
      }
      .popover-body {
        min-width: 276px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <div style="display: none;">
      <!-- Clickable label for Vienna -->
      <a class="overlay" id="vienna" target="_blank" href="https://en.wikipedia.org/wiki/Vienna">Vienna</a>
      <div id="marker" title="Marker"></div>
      <!-- Popup -->
      <div id="popup" title="Welcome to OpenLayers"></div>
    </div>
    <script src="main.js"></script>
  </body>
</html>
package.json
{
  "name": "overlay",
  "dependencies": {
    "ol": "6.8.1"
  },
  "devDependencies": {
    "parcel": "^2.0.0-beta.1"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --public-url . index.html"
  }
}