Short answer: Income doesn’t affect eligibility for employer or unsubsidized individual plans, but it determines eligibility for ACA premium tax credits, cost-sharing reductions, and Medicaid/CHIP.
Employer group coverage and unsubsidized individual coverage have no income test; anyone eligible can enroll regardless of earnings. Income matters only for assistance: it sets eligibility for ACA premium tax credits and cost-sharing reductions on the marketplace, and for Medicaid or CHIP. So while income never blocks you from buying coverage, it can determine how much help you get paying for it.