When I wipe the internal storage using TWRP recovery, does this fully wipe the contents of the phone's hard disk, leaving a blank, unformatted space? After doing this, will I be able to restore the stock firmware? I have two devices: a Nexus 7 2013 WiFi (flo) and a Moto G 2014 XT1068 (titan).
As for your first question: no, wiping data from the recovery just wipes the content of the partitions, not the partitions themselves. So your device's disk will still be divided into /data, /system, /cache etc., but those partitions will be blank.
As for your second question, yes, you will be able to restore the stock firmware, as long as you make a backup in the recovery onto an external SD card.