This is my understanding, as someone married to a healthcare professional and with sick parents I've followed it reasonably well.

Before the ACA went into effect, the main focus that everyone talked about was on the individual insurance market: in particular, trying to fix the adverse selection issues that had kept the individual insurance market in a continuous death spiral. ACA tried to fix that with the "Three Legged Stool": guaranteed issuance (no longer could a pre-existing health issue be denied coverage), government subsidies for people who couldn't afford it, and the individual mandate (everyone has to buy insurance or pay a fee) to make sure people didn't free-ride while healthy and only get insurance when they were sick. There was also a side project, which was this was that subsidies for really poor people turned out to be really expensive, so perhaps it would be easier to have everyone below a certain threshold just get Medicaid (government insurance for poor people that for complicated historical reasons is largely federal but partly each individual state funded- this is distinct from Medicare, which is entirely Federal and offers universal coverage to everyone over 65). So everyone can use the exchanges to get insurance, if they don't get insurance they have to pay a fee in their taxes at the end of the year, and Medicaid had expanded eligibility rules to cover more people. Health care wonks largely expected the exchanges to do most of the work, and some of them even envisioned a future where the link between jobs and health care were broken- your job pays you more money and you get insurance on your own through the exchange. But that was a future dream, for the ACA itself the focus was on just getting the exchanges to run. That is what healthcare.gov was all about, matching you to your local exchange and offering you the right plans and subsidy amounts- tying together insurance companies, the IRS for your tax returns, and state IT systems all together.

Then the conservative-controlled Supreme Court surprised many, and upheld the ACA exchanges on a 5-4 vote, though they added a random restriction never found before in any Supreme Court ruling that each state would have to accept the Medicaid changes on their own- a rewriting done by the Chief Justice's clerks in haste (after he apparently changed his vote late in the game to allow the rest of the law to stay, the liberal wing joined him immediately, no matter how weird it was, much better than losing the whole thing).

Now a decade on, the exchanges have proven to not be very popular- everyone who can still gets their insurance through their company because it is seen as a better deal than going through the exchange (because of corporate consolidation on both the insurance company and medical provider sides there is actually very little competition in the exchanges, so the prices didn't go down as expected). Far more people are covered by insurance than before, but it was almost entirely because of the Medicaid expansion- slowly over time even red states have gotten into it, and now only 10 states are still holding out, even as their rural hospitals are utterly devastated and many are forced to close because they can't get paid for their work (in order to balance the budget within the act, some of the US government programs that used to support poor people without insurance were ended- they'll all have Medicaid, so are unnecessary- except then the Supreme Court's rewriting meant that many poor people don't have Medicaid, and the GOP views even attempts to fix things for their own states as a step too far). The individual mandate- which had been viewed as essential to the success of the exchanges- was zeroed out by the Republicans when they took power, that is, the fee for not having insurance was set to 0. And it didn't really have much noticeable effect, because the exchanges didn't make much difference in the end, the power turned out to all be in the Medicaid expansion.

This in turn is what has driven the more liberal health care wonks towards single-payer and BernieCare and the like: if the exchanges didn't have much impact and Medicaid did, why not just make Medicaid-for-all?