Flash Storage and Flash Memory it's all in the name.
Easy
'''Flash Storage''' is on a separate card with a special Apple connector, we don't know when and if a third party will make an other size available.
'''Flash Memory''' this is soldered on the board. impossible to upgrade or change.
I really recommande to upgrade the Memory to 16GB. and for the storage, 512gb is good for me and if I need more space I use my 16TB NAS at home and I can access it form anywhere so I can stream stuff back to my MAC RETINA.