There are old stars called metal-poor. They are usually used as a tool to study the early universe. These stars are long-lived, so they have a low mass (0.6 - 0.8 mass of the Sun, lifetime of 15-20 billion years) and they are still observable in the Milky Way.

Our Milky Way, this is the bulge, the inner part of our galaxy with a supermassive black hole in the center. There are actually two disks in the either sides of the galaxy. The Earth is about 2/3 on the way out of a disk.

The bulge contains a lot of young stars and gas. Over time, more stars and elements are formed, resulting in a bulge that is very metal-rich. The disk is not quite as metal-rich but still pretty enriched.

This is just the most visible part, namely the Milky Way band on the night sky. Another definition to this part is the spiral arms that make up the disk.

For the older stars, because they are located up and below the disk in something that is called the halo of the disk. In the formation of a galaxy, smaller systems actually come together and form a bigger system, known as hierarchical structure formation paradigm. The largest one will be the Milky Way, while the smaller system have a tendency to end up in the outskirts. But they are completely shredded apart and what is left are all the stars that are being spilled into the Milky Way.