と言(い)うCalled, Named, Known as, As much as

Both expressions contain the same という, but are used differently. という is used to name or describe something, typically in the form AというB ('a B called A'). In contrast, というのは introduces something as a topic for explanation or comment, e.g. 人生というのは ('This thing we call life') While という is followed by a single noun, というのは is followed by a phrase that elaborates on the topic.

いわゆる and という both convey the idea of something being referred to by a name, but they function differently. いわゆる comes before a noun to mean 'the so-called (A)'. という appears between two nouns in the form AというB ('a B called A'). Because they serve different grammatical roles, they can appear together, e.g. いわゆる外来語というもの ('What is known as a so-called loanword')

ということ turns phrases into nouns and is often used to clarify or restate something, e.g. 日本に行くということ 'the fact that (he) is going to Japan'. という by itself links a phrase or name to a noun, as in ポチという犬 'a dog named Pochi' or 来年日本に行くという学生 'a student who is going to Japan'.

という is versatile: AというB = 'a (B) called (A)' or to show hearsay, e.g. 面白いという本 'a book said to be interesting'. といわれている is more formal and emphasizes that something is generally said or believed by many people, especially in written or academic contexts.