How to make Filter Aggregations using NEST?

I have a requirement for filter aggregations using NEST. But since I don't know much about this, I have made the below:

class Program
{
        static void Main(string[] args)
        {

            ISearchResponse<TestReportModel> searchResponse =
                            ConnectionToES.EsClient()
                            .Search<TestReportModel>
                            (s => s
                                .Index("feedbackdata")
                                .From(0)
                                .Size(50000)
                                .Query(q =>q.MatchAll())
                            );
                            
            var testRecords = searchResponse.Documents.ToList<TestReportModel>();
                
            result = ComputeTrailGap(testRecords);

        }
        
        
        private static List<TestModel> ComputeTrailGap(List<TestReportModel> testRecords)
        {
            var objTestModel = new List<TestModel>();          
            
            var ptpDispositionCodes = new string[] { "PTP" };
            var bptpDispositionCodes = new string[] { "BPTP","SBPTP" };
                  

            int gapResult = testRecords.Where(w => w.trailstatus == "Gap").Count();           

            var ptpResult = testRecords.Where(w => ptpDispositionCodes.Contains(w.lastdispositioncode)).ToList().Count();

            var bptpResult = testRecords.Where(w => bptpDispositionCodes.Contains(w.lastdispositioncode)).ToList().Count();
           

            objTestModel.Add(new TestModel { TrailStatus = "Gap", NoOfAccounts = gapResult });           
            objTestModel.Add(new TestModel { TrailStatus = "PTP", NoOfAccounts = ptpResult });
            objTestModel.Add(new TestModel { TrailStatus = "BPTP", NoOfAccounts = bptpResult });         

            return objTestModel;
        }
}

DTO

public class TestReportModel
{
   
    public string trailstatus { get; set; }        
    public string lastdispositioncode { get; set; }        
}

public class TestOutputAPIModel
{
    public List<TestModel> TestModelDetail { get; set; }    
}

public class TestModel
{   
    public string TrailStatus { get; set; }
    public int NoOfAccounts { get; set; }
    
}

This program works but as can be figure out that we are only accessing the Elastic Search via NEST and the rest of the Aggregations /Filter are done using Lambda.

I would like to perform the entire operation (Aggregations /Filter etc) using NEST framework and put it in the TestModel using filter aggregation.

How can I construct the DSL query inside NEST?

Update

I have been able to make the below so far but the count is zero. What is wrong in my query construction?

var ptpDispositionCodes = new TermsQuery
            {
                IsVerbatim = true,
                Field = "lastdispositioncode",
                Terms = new string[] { "PTP" },
            };
var bptpDispositionCodes = new TermsQuery
{
    IsVerbatim = true,
    Field = "lastdispositioncode",
    Terms = new string[] { "BPTP" },
};

 ISearchResponse<TestReportModel> searchResponse =
                ConnectionToES.EsClient()
                .Search<TestReportModel>
                (s => s
                    .Index("feedbackdata")
                    .From(0)
                    .Size(50000)
                    .Query(q =>q.MatchAll())
                    .Aggregations(fa => fa

                        .Filter("ptp_aggs", f => f.Filter(fd => ptpDispositionCodes))
                        .Filter("bptp_aggs", f => f.Filter(fd => bptpDispositionCodes))

                        )
               );

Result