Kotlin has a complete and elegant type hierarchy – if that is the right word, since it is a directed acyclic graph rather than a tree – described with diagrams at http://www.natpryce.com/articles/000818.html and https://blog.kotlin-academy.com/the-beauty-of-kotlin-typing-system-7a2804fe6cf0
What I'm not clear about is how generics fit into all this.
How do generic types like List and Map, and specializations thereof like List<Int> and Map<String, Int> fit into the type hierarchy?
List and Map are not types; they're classes. To form a type, you need to specify the necessary type parameters, e.g. List<Int> (or a type projection such as List<*>).
(This is different from Java, which allows raw types, mainly for backward compatibility.)
The type relationships depend upon the variance: for example, List<Int> is a subtype of List<Number>, because it's covariant in its type parameter; but MutableList<Int> is invariant and has no relationship with MutableList<Number> (except that they're both subtypes of Any).
This adds extra dimensions to the type hierarchy, making it rather more complex than the simple graph you may have seen. But the principle's straightforward enough, so it's not usually drawn in.
More details, as ever, in the Kotlin documentation.
