الَّذِي، الَّتِي are relative nouns identified by a following relative clause (ṣila).
جَاءَ الَّذِي نَجَحَ. — The one who succeeded came.
Alladhī: relative noun built on sukūn, in nominative position as fāʿil; najaḥa: past verb whose subject is an implied huwa. The relative clause has no independent iʿrāb position.
The relative noun’s built form remains fixed.