Two ways plans share cost
A copay is a flat fee: $25 for primary care, $50 for a specialist. You know the number before you walk in.
Coinsurance is a percentage: you pay 20%, the plan pays 80% after you meet your deductible. The actual dollar amount depends on the negotiated rate, not the sticker price.
Which applies when
Many plans use copays for office visits even before the deductible is met. Hospital stays and imaging often require you to hit the deductible first, then pay coinsurance.
Read the common services table on your SBC. It lists each scenario in plain rows.
A worked example
Suppose your deductible is $2,000 and coinsurance is 20%. An MRI with a negotiated rate of $1,200 applies $1,200 to your deductible. You pay the full amount if you have not met the deductible yet.
Later in the year, after the deductible is satisfied, the same MRI might cost you $240 (20% of $1,200) while insurance pays the rest.