It’s the card itself fairly similar to how controllers work in HDD/SSDs basically they abstract the physical storage medium which allows them to use a wide range of flash memory on the backend while maintianing compatibility with the SD format.

While you can achieve the same abstraction in hardware it’s much easier and cheaper to simply pick a small micro controller and do all the black magic you need in software especially since the flash on some of these cards can be really really bad as it’s often the lowest grade flash or worse recycled memory that ends up being used which means you end up with chips that are 50-80% defected so the controller ensures that these sectors/cells aren’t used.