There are two alternatives to traditional Marketplace plans: Health Sharing plans and Short-Term Major Medical plans.
Health Sharing plans are NOT insurance, strictly speaking, but do satisfy the ACA requirement. As a member of a Health Sharing plan you pay a monthly “contribution” instead of a premium. When you see your doctor, they will send the bill to the Health Sharing company who then pays the bill using money from a pool of every member’s contributions.
Short-Term Major Medical plans are designed to last no more than 90 consecutive days. They are helpful for providing coverage while someone is in between jobs, until someone becomes eligible for Medicare, or if someone missed the chance to enroll in an ACA plan. Some plans can be “stacked” to provide coverage longer than 90 days.