To the lower left we see the PSU mounting area, you can use ATX PSUs up to 175 mm deep when using the bottom fan location. We'll show you that later on during installation though.
As stated before, the chassis comes with a good seven expansion slots. CPU coolers may be up to ~160 mm tall and graphics cards up to 28cm in length. Here's the backside. If you look upwards you can see a black metal plate on the backside of the motherboard tray (over the big gap) that can hold three SSDs. To your lower left there are bay compartment for HDDs/SSDs (three of each).
Simple pull the tray plastic a bit, slide the HDD in there and it is secured.
Hard to spot but here the HDD is inserted into the lower slot; just slide it in until it clicks secure. BTW - please don't mind the old damaged HDD, it's just a mock-up unit :) As you can see the cabling is all black as well as most of the visible connectors.
So this tray can hold 2.5" SSDs, three of them are included. You'll need to secure each SSD with four mounting screws. It's a very arbitrary simple solution really, but then again, ... there's nothing wrong with simplicity as long as it's functional and effective. And it certainly works just fine.