Short answer: Emergencies are covered anywhere in the U.S. For routine care while traveling, coverage depends on your plan type: many Blue Cross plans use the national BlueCard network, while narrow HMOs may only cover out-of-area emergencies.
Every ACA-compliant plan must cover emergency care at in-network cost-sharing no matter where you are in the country. The bigger question is routine or follow-up care away from home.
PPO and many national-carrier plans participate in nationwide networks (for example, the Blue Cross Blue Shield BlueCard program), so you can often see in-network providers in other states. HMO and EPO plans are usually built around a local service area, and non-emergency care outside that area may not be covered at all, or only as out-of-network.
If you split time between states, attend college elsewhere, or travel often, check whether your plan has a national network, what counts as your service area, and how out-of-area care is handled before you need it. For dependents living elsewhere (such as a child at college), ask the carrier how to find in-network providers in that location.