However, what you are seeing is probably someone using a library of some sort.
Specifically, avr-libc:
http://www.nongnu.org/avr-libc/user-manual/index.html
Since the Arduino libraries are built on top of avr-libc, they too are available for your use. Note that they aren't as "user-friendly" as the Arduino libraries (the Arduino libs consist of many wrappers which abstract a lot of work away, among other things), but many of the library functions (such as strlen) are simple enough to grasp, especially with an example or two.