The word "市中心" is formed by directly combining the character "市" (shì), meaning "city", with the character "中心" (zhōngxīn), meaning "center". This creates a compound where "市" acts as a specifying modifier, narrowing the general concept of "center" to the specific context of a city, thus yielding the meaning "city center" or "downtown". It is a straightforward semantic fusion where the individual meanings of the characters are retained and combined to express a more precise idea.