it's a dynamic resonance, not a rigid object
Nothing is "locked" in place, nothing is solid. The bar and the spiral arms are dynamic features - individuial stars move in and out of them but tidal resonances and interactions between stars, gas, and dark matter allow these dynamic associations to build up.
be aware that what we SEE is not the same as the actual mass distribution. We only see the concentrations of brighter, younger stars. This is especially true for the spiral arms and the excellent post of ER above links to this neat diagram showing how ordered orbits can create apparent spiral structures from random chaos...