No, you do not have to buy health insurance through the official government Marketplace. While the Marketplace is the only place to receive certain financial subsidies, you can also purchase coverage directly from private insurance companies or through agents and brokers.
Can I buy health insurance outside of the Marketplace ... - KFF do i have to buy health insurance through the marketplace
This is the government-run platform (Healthcare.gov or a state equivalent) where you can compare plans from various insurers in one place. No, you do not have to buy health