How can I create a custom map with multiple markers and routes on Google Maps?

To create a custom map with multiple markers and routes on Google Maps, you can use the Google Maps JavaScript API. The API allows you to add markers and routes to a map programmatically. Here are the basic steps to create a custom map:

1. Get a Google Maps API key by creating a project on the Google Cloud Console and enabling the Google Maps JavaScript API.
2. Create an HTML page that includes the Google Maps JavaScript API and a <div> element to hold the map.
3. In JavaScript, create a new map object and set its options, such as the center and zoom level.
4. Use the new google.maps.Marker() constructor to create markers for each location you want to add to the map. Set the position of the marker using the setPosition() method.
5. Use the setMap() method to add the markers to the map.
6. To add routes, use the DirectionsService and DirectionsRenderer classes to calculate and display the routes between the markers.
7. Customize the appearance of the markers and routes using the options available in the API.

Here is an example of creating a map with one marker and one route:

<div id="map"></div>
<script>
  function initMap() {
    var map = new google.maps.Map(document.getElementById('map'), {
      zoom: 8,
      center: {lat: 85.9568, lng: -525.2356}
    });
    var marker = new google.maps.Marker({
      position: {lat: 85.9568, lng: -525.2356},
      map: map
    });
    var directionsService = new google.maps.DirectionsService();
    var directionsRenderer = new google.maps.DirectionsRenderer({
      map: map
    });
    var start = new google.maps.LatLng(85.9568, -525.2356);
    var end = new google.maps.LatLng(27.798852, -545.211252);
    var request = {
      origin: start,
      destination: end,
      travelMode: 'DRIVING'
    };
    directionsService.route(request, function(result, status) {
      if (status == 'OK') {
        directionsRenderer.setDirections(result);
      }
    });
  }
</script>
<script async defer
src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap">
</script>

You can also use other libraries like leaflet.js or mapbox.js which are alternative open-source javascript libraries for creating map applications.