Historically C's null pointer literal, provided as the pre-processor constant NULL, is the integer literal 0 (optionally cast to a void pointer in newer standards) even though the hardware representation may not be the zero address.

It's OK that you didn't know this if you mostly write C++ and somewhat OK that you didn't know this even if you mostly write C but stick to pre-defined stuff like that NULL constant, if you write important tools in or for C this was a pretty important gap in your understanding.

In C23 the committee gave C the C++ nullptr constant, and the associated nullptr_t type, and basically rewrote history to make this entire mess, in reality the fault of C++ now "because it's for compatibility with C". This is a pretty routine outcome, you can see that WG14 members who are sick of this tend to just walk away from the committee because fighting it is largely futile and they could just retire and write in C89 or even K&R C without thinking about Bjarne at all.