I don’t understand fusion drives. Can someone please tell me?

A Fusion Drive combines a small SSD and a large HDD. The idea was to strike a compromise that offered some of the advantages of a SSD (speed) without completely sacrificing the advantages of a HDD (capacity, cost per byte).

Physically,

• The HDD is a standard 2.5" SATA notebook drive or 3.5" SATA desktop drive.
• The SSD can be soldered to the motherboard, or on a plug-in module. A plug-in module might be of an Apple-specific type. On some Macs for which SSDs and Fusion Drives were an option, Apple only included the part of the main board needed to plug in a SSD if you ordered a SSD or a Fusion Drive from the factory. If you bought a machine with just a HDD, there was no place to plug in a SSD later.

If the Fusion Drive is operating correctly, the SSD will not show up as a separate drive in the Finder.

macOS creates a logical drive, in software, whose contents are spread across the SSD and the HDD. The SSD is not merely a cache for the hard drive. Some things may be stored only on the SSD, others only on the HDD, and should you break the connection, or remove either physical half, you destroy the integrity of the Fusion Drive. It's completely up to macOS where to put things – there are no manual controls to force macOS to put something on the SSD or to put it on the HDD.