﴿لَهُمْ فِيهَا دَارُ الْخُلْدِ﴾
For them, within it, is an abode of permanence. (Fuṣṣilat 41:28)
The antecedent of “fīhā” is Hell. Hell itself is the place of enduring punishment, yet the expression mentions a distinct “abode of permanence” within it, as though another lasting residence had been drawn from Hell.
Source: Hell, referred to by “fīhā.”
Extract: “Dār al-khuld.”
Common feature: abiding or permanence.
Fī establishes the locative relationship that permits this conceptual extraction. The verse is not asserting a physically separate architectural residence inside Hell.