They have a special OS driver which moves the FTL closer to the OS. So one of the things they need is multi GB of memory from the OS to keep the FTL mapping tables. Also sudden power loss requires OS intervention to piece back the FTL structure (I seem to recall the original product taking 5 minutes to recover.) This also means you couldn't boot from an Fusion IO drive. I'm not sure if they fixed these issues on a more recent drive.
They use flash like all other SSDs, but don't use a disk controller or any community defined protocols for mass compatibility. They use their own software and an FPGA to control the flash.
fusion-io (i believe, but please verify online) uses a spinning drive for frequent writes and an ssd for frequent reads, with software deciding what goes where, lessening the write traffic to the ssd, and thus wear to it.
No, Fusion-io has nothing to do with spinning drives. They make PCI-e flash drives with an FPGA as the "controller" for the flash. There are multiple communication channels, so you can simultaneously read and write from the drives, and there are various tunings available to control how garbage collection works. They are the only "SSD" that doesn't hide all the flash behind any kind of legacy disk controller or group protocol like NVMe
I think what you're trying to describe is apple's fusion drive.