[ApiController]
[Route('api/mainroute')]
public class MyControllerClass ...

    [HttpPost('my/additional/subroute1')]
    public async Task<IActionResult> MyEndpoint1Async ...

    [Authorize(Policy = PfwPolicyRequirement.EditorOrAdmin)]
    //[ProducesDefaultResponseType] todo what is this ?
    [ProducesResponseType(StatusCodes.Status204NoContent)]
    [ProducesResponseType(typeof(IReadOnlyCollection<string>), StatusCodes.Status400BadRequest)]
    [ProducesResponseType(typeof(IReadOnlyCollection<string>), StatusCodes.Status404NotFound)]
    [Produces("application/octet-stream")]
    [HttpGet('my/additional/subroute2/{id:Guid}', Name = 'anothername')]
    public async Task<IActionResult> MyEndpoint2Async ...

var presentationAssembly = typeof(PresentationProj.AssemblyReference).Assembly;

// AddControllers() needs <FrameworkReference Include="Microsoft.AspNetCore.App" />
services.AddControllers().AddApplicationPart(presentationAssembly);

<PackageReference Include="Microsoft.Extensions.Hosting.Abstractions" Version="8.0.0" />
<PackageReference Include="Microsoft.AspNetCore.Components" Version="8.0.5" />

[HttpGet]
[ProducesResponseType(StatusCodes.Status200OK)]
[Produces("application/octet-stream")]
[Route("download0", Name = "zipfile")]
public async Task DownloadBots()
{
    Response.ContentType = "application/octet-stream";
    Response.Headers.Add("Content-Disposition", "attachment; filename=\"Bots.zip\"");

    var botsFolderPath = Path.Combine("c:\\temp", "bots");
    var botFilePaths = Directory.GetFiles(botsFolderPath);
    using (ZipArchive archive = new ZipArchive(Response.BodyWriter.AsStream(), ZipArchiveMode.Create))
    {
        foreach (var botFilePath in botFilePaths)
        {
            var botFileName = Path.GetFileName(botFilePath);
            var entry = archive.CreateEntry(botFileName);
            using (var entryStream = entry.Open())
            using (var fileStream = System.IO.File.OpenRead(botFilePath))
            {
                await fileStream.CopyToAsync(entryStream);
            }
        }
    }
}

[HttpGet]
[ProducesResponseType(StatusCodes.Status200OK)]
[Produces("application/octet-stream")]
[Route("download1", Name = "mycsv")]
public void DownloadCsv()
{
    Response.ContentType = "application/octet-stream";
    Response.Headers.Add("Content-Disposition", "attachment; filename=\"mycsv.csv\"");

    using (var streamWriter = new StreamWriter(Response.BodyWriter.AsStream()))
    using (var csvWriter = new CsvWriter(streamWriter, CultureInfo.InvariantCulture))
    {
        csvWriter.WriteRecords<User>(users);
        csvWriter.Flush();
    } // StreamWriter gets flushed here.
}

[HttpGet]
[ProducesResponseType(StatusCodes.Status200OK)]
[Produces("application/octet-stream")]
[Route("download2", Name = "usercsvfile")]
public IActionResult Csv()
{
    var builder = new StringBuilder();
    builder.AppendLine("Id,Username,Email,JoinedOn,SerialNumber");
    foreach (var user in users)
    {
        builder.AppendLine($"{user.Id},{user.Username},{user.Email},{user.JoinedOn.ToShortDateString()},{user.SerialNumber}");
    }
    return File(Encoding.UTF8.GetBytes(builder.ToString()), "text/csv", "users.csv");
}

    public class User
    {
        public int Id { get; set; }
        public string Username { get; set; }
        public string Email { get; set; }
        public string SerialNumber { get; set; }
        public DateTime JoinedOn { get; set; }
    }

    private readonly List<User> users = new()
    {
      new User
      {
          Id = 1,
          Username = "ArminZia",
          Email = "armin.zia@gmail.com",
          SerialNumber = "NX33-AZ47",
          JoinedOn = new DateTime(1988, 04, 20)
      },
      new User
      {
          Id = 2,
          Username = "DoloresAbernathy",
          Email = "dolores.abernathy@gmail.com",
          SerialNumber = "CH1D-4AK7",
          JoinedOn = new DateTime(2021, 03, 24)
      }
    };