Your address on file with the Social Security Administration. Where you pay state taxes or are registered to vote. Where you spend the majority of your time annually.
If you live in two states (e.g., "snowbirds") or travel frequently, you still buy insurance in your primary state but should look for specific plan types that offer broader networks: How to report a move to the Marketplace | HealthCare.gov can i buy health insurance in another state
Under current federal and state laws, you generally purchase a health insurance plan in a state where you do not have a permanent residence. Insurance is regulated at the state level, and most individual plans require you to enroll in a plan available in the ZIP code of your primary residence . 1. Residency and Eligibility Your address on file with the Social Security Administration