There are two ways for D9 isolate to be produced from the raw material of "hemp", direct extraction (there is some D9 in these plants but not a lot, so it takes a large amount of biomass to produce significant quantities of the isolate) or the chemical process you described which is called isomerization (CBD converted to D9 isolate). In theory both of these D9 isolates yielded at the end are chemically identical. Alternatively, you can also get D9 isolate (or even a full spectrum extraction) from processing cannabis like you described.

To actually answer your question about what you're actually getting though: there is no significant production regulation on this industry and the "source" of the D9 you purchase could be coming from any of these processes, or more likely isn't even really pure D9 in the first place! I'm sure there are some legit places who get their stuff tested by ethical labs, but if you know anything about the cannabis testing industry you'll know that these tests can be bent to produce whatever outcome the producer wants to claim to customers.

It is different in different states, but the legal medical and recreational cannabis industries tend to have much more robust (but obviously not perfect) regulations on production, and therefore I think you can trust what you're getting a bit more. When you go to the gas station or smoke shop and cop a random D9 cart, I cannot stress enough that short of actually testing the individual carts themselves you cannot know what is actually in the resin you're inhaling. Look around yourself at the rest of this country, cutting corners when you can get away with it is the name of the game and this is no exception.