To understand what's going on here you need to understand drug formularies and pharmacy benefits managers (PBMs).
A PBM is an organization whose job is to negotiate prices for their clients, creating a catalog of drugs. Their client is usually not you, it's the insurance carrier. You may have seen their names on your plan's card or paperwork. The big ones are ExpressScripts, Caremark, Optum. To make things even more complicated, they are often owned by insurance companies, but tend to operate as their own divisions.
So the normal flow is:
1. PBM develops drug formulary by negotiating with drug manufacturers, pharmacy distribution networks
2. Insurance company hires PBM
3. Insurance company and PBM develop plan benefits, saying plan members will pay, for example, $10 for prescription drugs
4. Pharmacies, PBM, and insurance plan determine who gets how much for every prescription filled
5. Consumer shows up at pharmacy, shows insurance card, drug formulary and prices pull up on the pharmacist's screen, consumer pays price that's been agreed upon by everyone involved so far
Enter GoodRX and similar coupon apps. They realized that the way many insurance plans are structured can be a very bad deal for consumers. Maybe a drug isn't covered by the drug formulary at all, or maybe it's covered at too high a price.Recall that pharmacies, manufacturers, PBMs, and insurance companies all split the proceeds.
GoodRX, on the other hand is essentially a standalone PBM that markets directly to consumers, cutting out the insurance company. When you use their coupons, it's their pre-approved pricing that kicks in, not the insurance company.
Note that none of this has anything to do with the ACA. This is how pharmacy benefits work for everybody, not just ACA plan members.