function getApplicationFolder_() {

  // Gets root folder, currently set to 'My Drive'
  const parentFolder = DriveApp.getRootFolder();

  // Iterates through the subfolders to check if folder already exists.
  const subFolders = parentFolder.getFolders();
  while (subFolders.hasNext()) {
    let folder = subFolders.next();

    // Returns the existing folder if found.
    if (folder.getName() === APP_FOLDER) {
      return folder;
    }
  }
  // Creates a new folder if one doesn't already exist.
  return parentFolder.createFolder(APP_FOLDER)
    .setDescription(`Main application folder created by ${APP_TITLE}.`);
}
Returns the primary folder as named by the APP_FOLDER variable in the Code.gs file.
Checks if folder already exists to avoid duplication.
Creates new instance if existing folder not found.

@return {object} Google Drive Folder