I know that modern CPUs themselves often have integrated microcontrollers for things like the management engines and whatnot. In the thread about the M1 GPU it's noted that the GPU is controlled by a full ARM core. And then of course I'd assume there are a bunch on the motherboard and all of the I/O systems. Going back to even the Macintosh SE, its ADB implementation used a PIC microcontroller instead of a dedicated ASIC. I assume the number has exploded since then.