We live on the border of 2 states. We live in Michigan but all our doctors are in Ohio. Is ACA state by state? Can we live in Michigan and get insurance that will cover our Ohio doctors?
Insurance purchased directly by an individual anywhere, via the ACA exchange or not, is going to be location-specific. You need to identify what zip code you live in, which determines your county, which determines your state. Insurance plans that are available to be sold to you are based on this location. The "in-network" providers will vary from company to company and even plan to plan in many cases. Many of the large companies that provide nationwide insurance via employers do not sell plans directly to individuals, let alone via the ACA exchange. But you have to check who sells insurance in your home zip code.
Now that does NOT necessarily mean you cannot use medical providers in another state under your coverage. But it could mean that. You need to fully understand what the plan you purchase covers. They may or may not include out-of-state providers.
So, if you've decided you want to purchase off the ACA exchange (which you must do if you wish to get an ACA subsidy), then you need to comb through the available plans and see what they will cover. You look up specific doctors to see if the plan covers them. You look up specific hospitals to see if the plan covers them.
If your plan is a PPO, then it probably has some coverage in other states, but it may be at higher out-of-network pricing.
I'm 10 miles from Ohio, living in PA. My ACA insurance plan has a list of about 50 hospitals that are in-network. Most of them are owned by one corporate hospital system. I don't believe any of them are across the border in OH, but I haven't actually looked that up since I have a PA one that is closer.
Edited to add: I just looked at the network supported by my plan. There are indeed a few hospitals across state borders that are included in my plan -- several Cleveland Clinic hospitals. But certainly not all hospitals, just a select few. I'd have to check individual doctors by name to know if they were in-network or not.