Most of it is just the shape of the eye (spherical with a concave cornea),
the final fine focus is an opposing set of muscles which distort the shape of the lens.
You can adjust the focus for each eye separately.
the lens
A poet's eye would focus.
eye piece, lens and fine adjustment, focus adjustment
by moving and looking at it
pupil
the retina
No, to focus ligh, muscles in the eye change the length and thickness of the lens.
The pupil or cornea
pupil
Keep your eye on it and focus
The iris