Since there is extra gas, most star formation occurs in the arms. Since high luminosity O and B stars explode in a short time, they don't have time to leave the spiral arm. Thus, spiral arms are where all of the very brightest stars are. These are probably stars that formed together and haven't yet dispersed. The halo has many fewer stars.
Many halo stars are in globular clusters.
Halo stars orbit radially, while disk stars orbit in roughly circular orbits within the disk. The bulge is a combination of young and old stars. Active star formation is occuring, but most of the stars are very old. We estimate the mass of the Milky Way from the orbits of the Sun and other stars. We would expect this to be all of the mass of the galaxy, since there are far fewer stars outside of the Sun's orbit.