They have a secondary motherboard that hosts the Slot CPUs, 4 single core P3 Xeons. I also have the Dell equivalent model but it has a bum mainboard.
With those 90's systems, to get Windows NT to use more than 1 core, you have to get the appropriate Windows version that actually supports them.
Now you can simply upgrade from a 1 to a 32 core CPU and Windows and Linux will pick up the difference and run with it.
In the NT 3.5 and 4 days, you actually had to either do a full reinstall or swap out several parts of the Kernel to get it to work.
Downgrading took the same effort as a multicore windows Kernel ran really badly on a single core system.
As for the Sun Fires, the two models I mentioned tend to be highly available on Ebay in the 100-200 range and are very different inside than an X86 system. You can go for 400 or higher series to get even more difference, but getting a complete one of those can be a challenge.
And yes, the software used on some of these older systems was a challenge in itself, but they aren't really special, they are pretty much like having different vendors RGB controller softwares on your system, a nuisance that you should try to get past.
For instance, the IBM 5000 series raid cards were simply LSI cards with an IBM branded firmware.
The first thing most people do is put the actual LSI firmware on them so they run decently.