Why do some library names end with .h and some without?

Question

C++ / C#

Why do some library names end with .h and some without?

Answer 1

According to the classics, the idea was this - what is part of C ++ goes without .h
(iostream, map, vector, etc)
But then Qt and others came along, and decided to mix the cards by putting on spacer files without an extension that lead to files with extension.