Earlier quoted context omitted.
Embedded micro controllers are very common these days in a wide range of ICs most of them aren’t disclosed or accessible to their users their sole role is to abstract the physical IC and present what the customer expects this can be things like timers, flash memory or even microcontrollers themselves as well as more complex ICs that need to ensure some defined PHY like network or serial controllers.
That's what I expected but it sounds like these are specifically not that?
Which ones? The ones on SD cards are there for one reason only and that is to provide the hardware abstraction and PHY compatibility to allow SD cards to be manufactured more cheaply.
I haven’t seen any evidence that any of these MC does anything beyond that.