I am a little confused about these terms from the developer and user points. 
For example, I am on Ubuntu.

1. `GNU libs`(<https://www.gnu.org/software/libc/>) is installed by default, `gnulib`(<https://www.gnu.org/software/gnulib>) is not, right?
2. `GNU libs` follows POSIX stadard, but: 
<http://www.gnu.org/software/libc/manual/html_mono/libc.html#POSIX-Threads> ,
so looks like, for example, `pthread_create` is not implemented here and implemented in `Gnulib`, right?
3. `Gnome glib` is just a 3rd library, right? But it based on `GNU libs` or
`Gnulib`? Are there any other similar libraries?

Thanks.