An L-shaped or corner desk almost never fits through a door assembled, so the whole strategy is splitting it into its two or three sections at the corner joint. Once apart it is just a couple of ordinary desks.
Pro tip: Label the sections left, right, and corner, since L-desks only reassemble in one orientation and mixing them up wastes time.
Watch out: Do not try to force an assembled L-shaped desk through a doorway; it rarely fits and you will damage the joint or the door frame.