If you want to display the main location on the map, go to Locations and edit the location. In the views tab, make sure to check Display main location on the map. You also need to select a main location icon.

You can visit https://iconscout.com/unicons/explore/line to search for an icon for the main location, something like Pin or Home.

How to style the main location?

There is a unique class for every icon on the map, the class is formatted this way:

nbp-location-X-icon

where X is the ID of the location. To obtain this ID, look at the URL when editing a location.

[Your-Site]/wp-admin/term.php?taxonomy=nearby-places-location&tag_ID=32&....

The tag_ID can be used here as the ID.

You can add an style like this:

/* change the background colour */
.nbp-location-32-icon {
  background-color: #ff0303 !important;
}

/* change the icon colour */
.nbp-location-32-icon svg path {
  fill: #ffffff !important;
}

And your main location icon will be changed to something like this:

You can read this article for adding custom CSS to your site:

How to add custom css to WordPress