They're still using HDD's, so I'll simply take a guess that the HDD remains an "off-the-shelf" SATA 2.5" drive. I could be wrong.
Insofar as RAM is concerned, the Mini's enclosure looks all-but unchanged (except for the change of the firewire port to a second thunderbolt port), and I would reckon that the bottom still has the circular plastic plate that comes right off.
If there are not still two DIMM slots inside, I'll guess that that sub-assembly that previously contained the RAM slots, now is a sub-assembly with soldered-in RAM in place of those (previous) slots....
I'm starting to wonder on this too. The theme of this gen seems to be the reusing of old parts. Why make a whole new/different mobo for what feels like a side grade? Perhaps they just want to discourage our opening it up.