Download the Grabba app sample for iOS which is called GRCatalog.
Please check how we read the MRTD option in the GRCatalog Documentation. You will find very useful code samples here.
If you want to retrieve specific data from your ID document, you need to pass the fileID that corresponds to that specific data.
For instance, we pass 0x0102 to getDataFromMRZ() method in order to retrieve the face image data.
Please have a look at the ICAO eMRTD standard to find the specific data you need.