Basicly you are looking at two requests. The first request to find the directory file id (parent id). The second request to find the files within that directory.
Get the folder with the name of A:
var fileListRequest = service.Files.List();
fileListRequest.Q = "mimeType = 'application/vnd.google-apps.folder' and name contains 'A'";
var fileListResponse = fileListRequest.Execute();
Get all the files with a parent id of the previous request.
var filesInParent = service.Files.List();
filesInParent.Q = $"parents in {fileListResponse.Files.FirstOrDefault().Id}";
var allFilesInParentResponse = filesInParent.Execute();
Issues will arise with this if you have more then one directory with the name of A you will probably need to check its parent directory in that case to ensure that you have the proper directory that you are looking for.
Just trick is to check for mimeType = 'application/vnd.google-apps.folder'
that means that its a folder.
documentation on search
Some background info on search can be found here
List all files on Google drive
An alternate solution would be to just list all of the files on your google drive and sort though it locally. Due to the nature of the page streamer i would not recomend this if you have a lot of files on your drive account as each fetch is going to eat one of your quota But if your interested i have a tutorial on how to do it. List All files on Google drive