What's the difference between “C system calls” and “C library routines”?

Question

There are multiple sections in the manpages. Two of them are:

2     Unix and C system calls
3     C Library routines for C programs

For example there i

Answer 1

Libraries of common functions are built on top of the system call interface, but applications are free to use both.

System calls are like authentication keys which have the access to use kernel resources.

Above image is from Advanced Linux programming and helps to understand how the user apps interact with kernel.