For example, to express possession, in English we say "I have a pen", while in Russian we say "У меня есть ручка" (lit. "A pen is near me"), while in Latin we say something similar to Russian but use dative for possessor ("Stilus mihi est", lit. "A pen is to me"). Interestingly, all three languages have the grammatical and lexical tools to put any of the three phrases together (English, Russian and Latin all have a verb for "have", a verb for "be", dative case/preposition and a way to express proximity), yet each of the three languages is stuck with one primary way to express possession.
Or another example: expressing liking. Where English uses a transitive verb that governs the liker as a subject and the likee as a direct object ("I like cats"), Russian uses a reflexive verb that governs the likee as subject and the liker as an indirect object (lit. "Cats appeal to me"). And Japanese uses an adjective that attributes the likee, while the liker can be expressed with the thematic case if necessary (lit. "As for me, cats are likeable").
In other words, different languages prefer to use different sets of lexical and grammatical tools to express the same functional constructions.
I wonder what field of linguistics studies these differences in how languages put together different kinds of expressions and reusable constructions. Functional grammar isn't quite that (and too broad), pragmatics also isn't quite that.