What actually changes in a mind on the way to enlightenment?
Enlightenment is easy to talk about and hard to say anything precise about. Prajñāpāramitā is the attempt to be precise: not what enlightenment feels like, but what changes, in what order, and how one would know.
It occupies seven of the nineteen years — longer than any other subject in the curriculum.
The Perfection of Wisdom sutras are enormous; the longest runs to a hundred thousand lines. The Ornament of Realization (Abhisamayālaṃkāra), attributed to Maitreya and transmitted through Asaṅga in the fourth century, is a compressed outline of them — a table of contents that turned out to be more studied than the thing it indexes.
It is famously terse. Read alone it is close to unintelligible, which is precisely why it is taught through commentary and debate rather than by reading.
Monks approach the Ornament through its structure: eight subjects broken into what the tradition calls the Seventy Topics, and studied alongside the grounds and paths (sa lam) — the stages a practitioner passes through, each with its defining marks.
The effect is to turn something that could stay vaguely inspirational into something examinable. A monk can be asked which ground a described mind has reached, and must defend his answer.
Partway through — years seven and eight at Sera Jey — the curriculum breaks off to study Tsongkhapa on the Mind-Only school (Cittamātra), a Buddhist position the Gelug tradition does not finally accept.
Monks spend two years learning to state and defend a view they will later argue against. This is not an oversight. A position you cannot argue for is one you have not understood well enough to reject.
Back to education and intellectual training, or on to the other treatises: Pramāṇa, Prajñāpāramitā, Madhyamaka, Abhidharma, Vinaya.