Where your saved images are
Images you save in iDig appear on the folder tab at the bottom of the screen, grouped into one folder per page you saved from.
They are not added to the Photos app automatically
Images are stored in folders inside the app, so they don't show up in Photos on their own. To move them there, open the folder, select the images and choose "Save Image" from the share button. You can select several at once.
Getting them onto a computer
The same folders are visible in the iPhone Files app under "On My iPhone" → the app's name. From there you can copy them elsewhere or move them to a computer. This works even if the app won't start.
See also Where did that feature go?