Expansion slots
1 M.2 2230
1 M.2 2280
Note on expansion slots
1 M.2 slot for WLAN and 1 M.2 2280 slot for memory.

The following are plugged into the top of the main board
- Board for 2nd HDMI output approx. 37x32 mm
- 1024 GB approx. 80x22 mm horizontal
- WIFI module approx. 30x22 mm horizontal
- 2 SODIMM slots horizontal
- Socket 24-pin male (2x12) vertical, empty

I cannot answer this question as I have not opened the device and only use the external connections where there are enough available.

Thanks for the answers!
I hadn't realised that one slot is occupied by the WLAN module.

Note on expansion slots
1 M.2 slot for WLAN and 1 M.2 2280 slot for memory.

Good point, there was an issue. I had to leave it as it was and add a SATA disc. But my biggest problem was that the BIOS or whatever doesn't allow correct hierarchical virtualisation. So no VMs in a VMware VM.
Otherwise a great device