Well, here we go.
Compound nouns vs attributive nouns
Your research is not quite accurate. Your steel bridge is not really either and your teacup is a compound noun. However, steel is an attributive noun.
First off, what is a compound noun?
Well I leave it to Cambridge and just cite:
Some nouns consist of more than one word. These are compound nouns. Compound nouns can be formed in different ways. The most common way is to put two nouns together (noun + noun); other common types are adjective + noun and verb + noun.
Many compound nouns are written as one word, but some are written with hyphens or spaces. In modern English, hyphens are less common than they were in the past. A good learner’s dictionary will tell you how each compound is usually written.
You also have a look at the explanation on Wikipedia.
Essentially, a compound noun is a noun that is modified and still describes what nouns describe (the name of some specific thing or set of things, such as living creatures, objects, places, actions, qualities, states of existence, or ideas). The compound is comprised of all those words (e.g. science fiction writer, teacup, son-in-law).
Both sources also tell us that those words can be closed (one word), hyphenated or open (also called spaced) compounds which means several words (it's not limited to two; think science fiction writer). Obviously the latter are the tricky ones, since they sneak up on us. However, if you read through the sources I cited, you should get a decent picture.
Second off, what's an attributive noun?
An attributive noun is a noun that modifies another nouns. The noun is just the one that modifies the other noun as the wiki example shows us (noun adjunct is an alternative name for attributive noun):
For example, in the phrase "chicken soup" the noun adjunct "chicken" modifies the noun "soup". It is irrelevant whether the resulting compound noun is spelled in one or two parts. "Field" is a noun adjunct in both "field player" and "fieldhouse".
Lastly, the tricky part
Consider a science fiction writer and a romance writer, both are compound nouns. With the second it's simple , writer is the modified noun, modified by romance. Writer is the head of the compound (i.e. the thing at the core of it).
romance: attributive noun
romance writer: compound noun
With the science fiction writer it's a matter of perspective, when we are talking about the writer, science fiction is the noun adjunct. When we are just talking about science fiction, then only science is the noun adjunct modifying fiction. Thus, we have in fact a compound noun acting as noun adjunct to another noun, creating another compound noun.
science fiction (compound noun)
science (noun adjunct)
fiction (noun, head)
science fiction writer (compound noun)
science fiction (noun adjunct, compound noun itself)
writer (noun, head)