why web assembly can invoke c library function??

i have read the tutorial in official documentation, but cannot figure it out why ws supports c library like glibc stdlibc etc. From the doc I know that
1. C code can be converted to a wasm file which runs in a browser.
2. the wasm 's content is some cross-platform intermediate code.
now I have a main.c file like below
#include
int main () {
...
printf("helloworld");
..
}
my question is why printf works?? as I understand the c library binary code which including printf.o is not in my wasm file.