Confused about extension and shared_preload_libraries

Hi, hackers

When we write a extension using C language, we often add the dynamic library
into shared_preload_libraries, however, I found that the bloom, btree_gist and
btree_gin do not follow this rule.  I'm a bit confused with this, could anybody
explain it for me?

Thanks in advance.

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.