Is it true that there is not so thing as upper and lower abs? Will the bottom part of your abs only show if bodyfat levels are low?
Yes you will need low body fat to see lower abs
You have one set of abs, rectus abdominis. Upper and lower abs do work separately though. Getting ripped lower abs is down to body fat generally, unless you have no lower abdominal function.
I don't believe that it is debatable that the rectus abdominus is one large muscle group. Dissection of cadavers demonstrated this many years ago. EMG (muscle electrical activity) has shown that you can activate part of the muscle to a greater or lesser extent though. Whether this means that the less active part isn't under tension is still in question as you can't measure this.
In any case, you'll likely need to reduce bodyfat to see the lower abs as mentioned above.