A:Answer Up to 64GB of RAM, two additional NVME storage slots (I believe one is under the video card), and two tool-less, pre-cabled HDD bays accessible from the back. There are an additional two SATA ports with no drive mounts.
There are 8 USB type A ports and an unused PCIe x4 slot.
Motherboard, case, PSU, are all standard PC form factors. Nothing proprietary here. You could probably fit a two-fan radiator in the top of the case if you wanted to upgrade the CPU.