Understood. But the deductible does apply before the insurance pays on many plans.
Also, coinsurance is often paid AFTER, not before the deductible. Copays (on office visits) are often paid before the deductible but coinsurance may still apply on some plans (not many).
So they really are not applied to the deductible. They are separate. For example, if you have a $5,000 deductible with 20% coinsurance and max oop of $6,300, you will pay $1,300 in coinsurance AFTER the deductible has been reached (assuming a major claim).
I realize it's confusing. That's why I (and others) recommend you always work with an experienced broker. An inexperienced person may not accurately explain your options.