Set the following to a variable:
retrievefiles.DownloadFile("https://maliksuhail.github.io/myprices/hello.txt",System.IO.Path.Combine(FileSystem.Current.AppDataDirectory,"my.txt"));
TO:
var filePath = System.IO.Path.Combine(FileSystem.Current.AppDataDirectory,"my.txt");
retrievefiles.DownloadFile("https://maliksuhail.github.io/myprices/hello.txt", filePath);
Then you can output to the debug console:
Debug.WriteLine($"File Path: {filePath}");
Now you can see exactly where the file is stored.