For IO, memory management and similar "low-level services" it's often fine to call the underlying operating system functions directly and with this also make use of more powerful features than the standard library can provide, such as async IO, virtual memory etc (or wrap those OS calls into your own cross-platform wrapper functions). There's also plenty of libraries which do this for you, like libuv (but integrating such big libs is often not trivial).

Same for string handling/processing, there are specialized libraries out there which do a specific job better than the rather generic standard library functions (this is also true for the C++ stdlib), one just has to find those libraries.

PS: I don't think that a "batteries included" standard library (like python has) would even make sense for C, such a library would need to be opinionated by definition, better to keep (too many) opinions out of the standard, as this road just leads to another C++ ;)