API controller that returns references to a ContentHub asset by assetId plus an example widget for use on a contentHub asset details page

ContentHubAssetUsageController.cs

using Sitecore.Abstractions;
using Sitecore.ContentSearch;
using Sitecore.ContentSearch.Linq;
using Sitecore.ContentSearch.Linq.Utilities;
using Sitecore.Data.Items;
using System.Collections.Generic;
using System.Linq;
using System.Web.Http;
using System.Web.Http.Cors;

namespace MyProject.Controllers.Api
{
	[EnableCors(origins: "*", headers: "*", methods: "*")]
	public class ContentHubAssetUsageController : ApiController
	{
		private readonly BaseSettings _settings;

		public ContentHubAssetUsageController(BaseSettings settings)
		{
			_settings = settings;
		}

		[HttpGet]
		public IHttpActionResult GetUsages(long assetId)
		{
			if (!_settings.GetBoolSetting("ContentHubCustomisations.EnableAssetUsageController", false))
			{
				return Unauthorized();
			}

			return Ok(GetRelevantItems(assetId).Where(i => i != null).Select(MapFields));
		}

		private IEnumerable<Item> GetRelevantItems(long assetId)
		{
			ISearchIndex index = ContentSearchManager.GetIndex(_settings.GetSetting("ContentHubCustomisations.IndexName"));
			using (IProviderSearchContext context = index.CreateSearchContext())
			{
				SearchResults<ContentHubImagesSearchResultItem> results =
					context.GetQueryable<ContentHubImagesSearchResultItem>()
					.Where(x => x.ContentHubAssetIds.Contains(assetId.ToString()))
					.GetResults();

				if (results.Any())
				{
					return results.Hits.Select(h => h.Document.GetItem());
				}

				return new List<Item>();
			}
		}

		private AssetUsage MapFields(Item item)
		{
			return new AssetUsage()
			{
				ItemId = item.ID.ToString(),
				ItemName = item.Name,
				ItemPath = item.Paths.FullPath,
				Language = item.Language.Name,
				Version = item.Version.Number
			};
		}
	}

	public class AssetUsage
	{
		public string ItemId { get; set; }
		public string ItemName { get; set; }
		public string ItemPath { get; set; }
		public string Language { get; set; }
		public int Version { get; set; }
	}
}

settings.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/"
			   xmlns:set="http://www.sitecore.net/xmlconfig/set/"
			   xmlns:role="http://www.sitecore.net/xmlconfig/role/">
  <sitecore>

    <settings>
      <setting name="ContentHubCustomisations.IndexName" value="sitecore_master_index" />
      <setting name="ContentHubCustomisations.EnableAssetUsageController" value="true" role:require="ContentManagement or Standalone" />
    </settings>
    
  </sitecore>
</configuration>

widget.html

<div class="panel-container form-horizontal panel-container--view-mode panel--shadowed skin-m-default custom-related-by-collection-panel"> 
    <h3 class="header-caption text-overflow-hidden flex-1">Asset usage in CMS</h3> 
    <div>
        <button class="btn btn-primary js-get-usage-button">Show usage</button>
    </div>    
    <div class="custom-cms-usage-results">
        
    </div>
</div>

widget.js

var self = this; 
 
var entityLoadedSubscription = options.mediator.subscribe("entityLoaded", function(entity) { 
    self.entityId = entity.systemProperties.id();   
});
 
$('.js-get-usage-button').click(function(event) {       
    showUsages('local', 'https://my-sitecore-management');     // This should point to your management server
});
 
function showUsages(title, baseUrl) {
  
    var url = baseUrl + "/api/ContentHubAssetUsage/GetUsages?assetId=" + self.entityId ;
 
    const maxResults = 50;
     
    $.get( url, function( data ) {
        if (data.length <= 0) {
            $('.custom-cms-usage-results').append(`<h4 style="margin-top: 5px;">${title}</h4><small>No usage of this asset found.</small>`);
            return;
        }     
 
        $('.custom-cms-usage-results').append(`<h4 style="margin-top: 5px;">${title} (${data.length})</h4>`);
         
        $('.custom-cms-usage-results').append(`<table class="table">` + data.slice(0, maxResults).map(function(item) { return ResultItem(item, baseUrl); }).join('') + `</table>`);
         
        if (data.length > maxResults) {
            $('.custom-cms-usage-results').append(`<small>Max ${maxResults} results are shown.</small>`);
            return;
        }   
 
        $('.js-get-usage-button').hide();
    });
}
 
// Template for result item
const ResultItem = (item, baseUrl) => `
 
    <tr>
        <td>
            <a href="${baseUrl}/sitecore/shell/Applications/Content Editor?id=${encodeURIComponent(item.ItemId)}&amp;vs=1&amp;la=${item.Language}&amp;sc_content=master&amp;fo=${encodeURIComponent(item.ItemId)}&amp;ic=People%2f16x16%2fcubes_blue.png&amp;he=Content+Editor&amp;cl=0" target="_blank">
                ${item.ItemName}             
            </a> <br/>
            <small>${item.ItemId}</small><br/>
            <small>${item.ItemPath}</small>
        </td>
        <td><small class="text-nowrap" title="Version">${item.Version}</small></td>
        <td><small class="text-nowrap">${item.Language}</small></td>
    </tr> `;

API controller can be called through /api/ContentHubAssetUsage/GetUsages?assetId=<myAssetId>
If you haven't set up api routing in your solution yet, see details here.
You'll also need to register the API controller with the DI framework through a IServicesConfigurator.