If your account in Outlook is an IMAP account, the folders are stored on the server, so subfolders that you have created should sync automatically to your Mac and iPhone. How they display can be modified by going to the Mail app, the main page, then tap edit and select the inboxes you wish to show. Then below that list should be the accounts you have on the device, and if you tap on the arrow to the right, it should display your folder list there and you can select a folder there. But I'm not aware of a way to always display more folders than your inboxes. That is for the iPhone using the default Mail app. Using the Outlook app may differ. I do not use it so I could not comment on that. Also, the Mac could be different, I'm not using one.