Delve into the theoretical underpinnings of transit$-$oriented development $($TOD$),$ emphasizing the integration of high$-$density, mixed$-$use development with public transit systems to promote sustainable urban mobility, reduce car dependence, and enhance urban livability. How do TOD theories inform land use planning, transportation policies, and urban design practices that support compact, walkable neighborhoods, improve transit accessibility, and foster vibrant, connected communities?