I think it this case everything is defined in the article, except for List, which is either an empty list [] or a non-empty list x:xs

Anything lowercase is a function/variable, and will appear somewhere on the left-hand-side of an equals.

Anything uppercase is a Type/constructor. In this case I think they've only used a Tree, which they defined themselves (either a Nil or a Node).

any and scanl would typically be imports, but they've defined these in full.

They also defined init themselves unrelated to the one in the stdlib.