Snippet from MvcCoreServiceCollectionExtensions to show how the ApplicationName in the environment controls where controllers are looked for

MvcCoreServiceCollectionExtensions.cs

public static class MvcCoreServiceCollectionExtensions
{
	public static IMvcCoreBuilder AddMvcCore(this IServiceCollection services)
	{
		if (services == null)
		{
			throw new ArgumentNullException(nameof(services));
		}

		var partManager = GetApplicationPartManager(services);
		services.TryAddSingleton(partManager);

		ConfigureDefaultFeatureProviders(partManager);
		ConfigureDefaultServices(services);
		AddMvcCoreServices(services);

		var builder = new MvcCoreBuilder(services, partManager);

		return builder;
	}

	private static ApplicationPartManager GetApplicationPartManager(IServiceCollection services)
	{
		var manager = GetServiceFromCollection<ApplicationPartManager>(services);
		if (manager == null)
		{
			manager = new ApplicationPartManager();

			var environment = GetServiceFromCollection<IHostingEnvironment>(services);
			var entryAssemblyName = environment?.ApplicationName;
			if (string.IsNullOrEmpty(entryAssemblyName))
			{
				return manager;
			}

			manager.PopulateDefaultParts(entryAssemblyName);
		}

		return manager;
	}
}