In the beginning, the gas came from the original formation of the universe and
was made of just Hydrogen & Helium.
The gas became gravitationally unstable and started to collapse.
Very dense clusters of stars, called Globular Clusters, formed.
These stars had few metals, and their kinematic distribution
reflected the proto-galactic cloud they formed in (spherical).
However, some net angular momentum existed in the cloud.
As the matter begins to draw together, it can't help but spin up.
The STARS formed in the HALO stay behind. But the gas interacts,
and gives up angular momentum through frictional loss. This
allows it to collapse inward on itself.
The majority of the gas falls in to form the BULGE, which will have
a higher rotation rate than the HALO (conserving some angular
momentum). Stars in the Bulge will also rotate with a single,
more preferred direction.
Stars which form now, which was 8-10 Billion years ago, will have a few
heavy elements in them, because the interstellar material they formed
from will have been enriched by the now dead, massive stars first
formed in the Globular Clusters of the HALO.
What remains of the gas after the BULGE forms, has the highest
angular momentum of all and is unable
to fall to the center. It gets spread out, very thin, in the DISK.
The DISK is the only place gas exists and the
only place stars can still be born today. This gas
has already been through at least one episode of stellar fusion, giving
it a fairly high abundance of heavier elements (C, N, O, Fe, etc).