Learn Turkish: Başarılı yüzücü altın madalya kazandı. - The successful swimmer won a gold medal.

The word comes from the verb root yüz- (to swim). We add the agentive suffix -ücü (which follows 4-way vowel harmony: -ıcı, -ici, -ucu, -ücü). This suffix takes a verb and turns it into a noun describing someone or something that regularly performs that action.

Does the suffix -(y)ıcı / -(y)ücü only refer to people?

No, it can also refer to objects, tools, or substances that perform the action. While yüzücü means "swimmer" (a person) here, it can also refer to a "float" or floating mechanism in technical contexts. Another common example is soğutucu (cooler/refrigerator) from soğutmak (to cool).

Why isn't it altın madalyası? Doesn't a noun describing another noun usually require a possessive compound?

Normally, yes. However, when a noun describes the material something is made of—such as altın (gold), gümüş (silver), or ahşap (wood)—it acts as a direct adjective. You don't use the possessive suffix on the second word, so it remains simply altın madalya.

Why is there no accusative case marker on altın madalya? Shouldn't it be altın madalyayı kazandı?