A lot of people write C in C++ mode just for convenience, you get things like const, but it's not the same as writing actual standards compliant C code.

If you need portable C code, forget about all that stuff. Forget about a lot of things since some compilers are truly atrocious.

Almost all C is valid C++, but the reverse isn't true.

Almost but not all. Restrict keyword, by example.

Also modern features like atomics and complex numbers are implemented as language features in C, but with template libraries in C++. Likewise type-generic math in C11 uses the C-specific _Generic keyword, but uses templates and function overloading in C++