I’m try to get all MobileApps together with their Detection Rules in just 1 call to GraphAPI Beta. Using PostMan, sending 1 GET request https://graph.microsoft.com/beta/deviceAppManagement/mobileApps?$filter=isof('microsoft.graph.win32LobApp')&$top=1000 would return you a list of Top 100 MobileApps in JSON format. Each object would have a Detection Rules, containing various data which is not really the case when doing the same in C# using the Graph Beta library(at least to my knowledge).

Here’s a method I wrote to try and get the assignment ids, detection rules and createdOn:

public async Task<List<GraphResponseModel>> GetAllWin32LobAppWithDetectionRules()
    {
        try
        {
            
            Win32LobAppRegistryDetection detectionRule = null;
            List<GraphResponseModel> graphResponseModelList = new();
            List<string?> idsFromAssignment = [];

            // Get all Win32LobApps
            var mobileApps = await _graphClient.DeviceAppManagement.MobileApps.GetAsync(context =>
            {
                context.QueryParameters.Filter = "isof('microsoft.graph.win32LobApp')";
                context.QueryParameters.Expand = new string[] {"assignments"};
            });

            foreach (var mobileApp in mobileApps.Value)
            {
                mobileApp.Assignments.ForEach(assignment =>
                {
                    idsFromAssignment.Add(assignment.Id);
                });
                // Cast the mobileApp to a Win32LobApp
                var win32LobApp = (Win32LobApp)mobileApp;
                
                _detectionRulesModel = null;
                
                foreach (var detection in win32LobApp.DetectionRules)
                {
                    if (detection.OdataType == "#microsoft.graph.win32LobAppRegistryDetection")
                    {
                        detectionRule = (Win32LobAppRegistryDetection)detection;
                        _detectionRulesModel = new DetectionRulesModel(detectionRule.OdataType, (bool)detectionRule.Check32BitOn64System, detectionRule.KeyPath, detectionRule.ValueName,detectionRule.DetectionType.Value.ToString(), detectionRule.Operator.ToString(), detectionRule.DetectionValue);
                    }
                    else
                    {
                        continue;
                    }
                    _createdOn = mobileApp.CreatedDateTime.Value.DateTime;
                   
                    graphResponseModelList.Add(new GraphResponseModel(idsFromAssignment, _detectionRulesModel, _createdOn));
                }
            }
            return graphResponseModelList;
        }
        catch (Exception e)
        {
            Console.WriteLine(e);
            throw;
        } 
    }

Unfortunately, I get a response back with the list containing the actual amount of software I own, but only the _createdOn field is populated, other fields are empty.