This is the 3648th most frequent Russian word.
строителей
"Строителей" - "of builders" (genitive plural of "строитель").
Here, 'строителей' is used in the genitive plural form of the noun 'строитель' which refers to 'builders', describing a group performing work on a specific site.
Группа строителей работает на этом участке.
A group of builders is working on this site.
In this sentence, 'строителей' is again in the genitive plural form, indicating possession of the work being observed by the subject.
Мы наблюдаем за работой строителей.
We are observing the work of the builders.
'Строителей' is used here in the accusative plural form, designating the builders as the direct object of the act of thanking.