The case supports 3x 2.5" SSD and 1x 3.5" HDD simultaneously.
On the backside of the motherboard, there are 2x tool-less 2.5" mounts:
The 3.5" drive tray also support one 2.5" SSD in the bottom while also using a 3.5" drive:
Pop 2 Air HDD/SSD installation
Tim