How to list all files in google drive in a folder with a set name?

Question

I have a folder in my drive which houses multiple "Client" folders. Each client folder has the same structure which includes a folder that contains completed invoices (called "Completed invoices").

I need a way to iterate through all the folders named "Completed Invoices" and list in a google sheet the spreadsheet Id's in those folders so I can loop through those files later and extract data from them.

Ive found multiple code sources that work but only on the first level - i.e. it will list the files in the agents/clients folder, but it will not go into the sub folders. E.g. see code below

function listFilesInFolder() {

   var sheet = SpreadsheetApp.getActiveSheet();
   sheet.appendRow(["Name", "Date", "Size", "URL", "Download", "Description", "Type"]);

    var folder = DriveApp.getFoldersByName("FOLDER ID GOES HERE");
    //Logger.log(folder);
    var contents = folder.getFiles();

    var cnt = 0;
    var file;

    while (contents.hasNext()) {
        var file = contents.next();
        cnt++;

        Logger.log(file);
        Logger.log(cnt);

            data = [
                file.getName(),
                file.getDateCreated(),
                file.getSize(),
                file.getUrl(),
,
            ];

            sheet.appendRow(data);



    };
};

Solution

Thanks everyone for your answers, but after some more searching I found my answer. For anyone who is looking for a similar solution - I found an excellent script here which provides a really robust output that I imagine could be used for a variety of different scenarios:

/** 
 * Google Apps Script - List all files & folders in a Google Drive folder, & write into a speadsheet.
 *    - Main function 1: List all folders
 *    - Main function 2: List all files & folders
 * 
 * Hint: Set your folder ID first! You may copy the folder ID from the browser's address field. 
 *       The folder ID is everything after the 'folders/' portion of the URL.
 * 
 * @version 1.0
 * @see     https://github.com/mesgarpour
 */

// TODO: Set folder ID
var folderId = 'My folder ID';

// Main function 1: List all folders, & write into the current sheet.
function listFolers(){
  getFolderTree(folderId, false);
};

// Main function 2: List all files & folders, & write into the current sheet.
function listAll(){
  getFolderTree(folderId, true); 
};

// =================
// Get Folder Tree
function getFolderTree(folderId, listAll) {
  try {
    // Get folder by id
    var parentFolder = DriveApp.getFolderById(folderId);

    // Initialise the sheet
    var file, data, sheet = SpreadsheetApp.getActiveSheet();
    sheet.clear();
    sheet.appendRow(["Full Path", "Name", "Date", "URL", "Last Updated", "Description", "Size"]);

    // Get files and folders
    getChildFolders(parentFolder.getName(), parentFolder, data, sheet, listAll);

  } catch (e) {
    Logger.log(e.toString());
  }
};

// Get the list of files and folders and their metadata in recursive mode
function getChildFolders(parentName, parent, data, sheet, listAll) {
  var childFolders = parent.getFolders();

  // List folders inside the folder
  while (childFolders.hasNext()) {
    var childFolder = childFolders.next();
    // Logger.log("Folder Name: " + childFolder.getName());
    data = [ 
      parentName + "/" + childFolder.getName(),
      childFolder.getName(),
      childFolder.getDateCreated(),
      childFolder.getUrl(),
      childFolder.getLastUpdated(),
      childFolder.getDescription(),
      childFolder.getSize()
    ];
    // Write
    sheet.appendRow(data);

    // List files inside the folder
    var files = childFolder.getFiles();
    while (listAll & files.hasNext()) {
      var childFile = files.next();
      // Logger.log("File Name: " + childFile.getName());
      data = [ 
        parentName + "/" + childFolder.getName() + "/" + childFile.getName(),
        childFile.getName(),
        childFile.getDateCreated(),
        childFile.getUrl(),
        childFile.getLastUpdated(),
        childFile.getDescription(),
        childFile.getSize()
      ];
      // Write
      sheet.appendRow(data);
    }

    // Recursive call of the subfolder
    getChildFolders(parentName + "/" + childFolder.getName(), childFolder, data, sheet, listAll);  
  }
};