Why learn C? (O'Reilly Radar)

Most of those have non-prefixed versions which you can get by including a header. For example _Bool has <stdbool.h>, which does #define bool _Bool. For _Alignof include <stdalign.h> and get alignof. <stdatomic.h> does the same for many other types.

In C anything that starts with an underscore followed by a capital letter, or anything which begins with two underscores, is reserved for future language extensions. That way, new extensions won't break existing code. If you want the intuitive names, include the headers.