I've seen various forms, some where you could identify standard MFM/RLL or IDE drives just bolted onto an interface card, some where the on drive controller was more integrated with the board, and some that looked completely custom (From HDD manufacturers obviously) In their last gasp of the early 90s they tended toward standard IDE.
They were popular in the late 80s because there were many machines like Tandys and Amstrads that were supplied all floppied out, 2 drive or tight single drive configs, as were IBM PCs and XTs with full height drives. So from the factory there was no obvious space for a hard drive in some instances. Also in that era there were a lot more proprietary configurations around still, which had either limited space or for AT class a limited BIOS. These machines although probably half of the market in the late 80s are rare now because lacking easy or more standard upgrade options, they were among the first to the landfill... or later recycling. Some of these might have had only 6, 10 or 20 BIOS HDD types, and when new you could maybe order the right hard drive for them, just 2 or 3 years down the road however, they might have become unobtanium. So an all in one solution was attractive even if they did have drive bays and a controller available.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.