And some, such as RISC-V, have no predefined stack pointer at all. (Push and pop are implemented by doing them manually; call is implemented by the jump instruction optionally taking an argument for a register to use as the stack.) C, at best, teaches you the C abstract machine, which supports both of these concrete implementations equally well.

Leaning how microcontrollers work is not the same as 'learning how the computer works' in general, which is the subject at hand.