This may differ from operative system, I am basically using Android for this.
The (usually) 16 character code isn't shown by the 2FA app - it is shown by the website or service you are setting up 2FA for. It should be shown alongside the QR code, or perhaps behind a "back up" button or similar. Only if the website doesn't show you the back up code (which should be a huge red flag in continuing to trust the security of that website) would you need to extract it manually from the QR code or from your 2FA app.
On Aegis, I also thought I have seen the secret character along with QR code before but it was not there when I checked it recently not quite long.
Long press on the entry you are interested in, click on edit, click on advanced, click on the reveal button next to the secret.