Can an iPhone really read the actual capacity (mAh) of the battery?

The iPhone 7 plus original battery has 2900mAh of capacity, I’d expect the software is calibrated accordingly. The only way to tell the real capacity is to discharge the battery down to its cutoff voltage and charge it via a good charger that will tell you how many mAh it was able to put into the battery. Modern chargers in the RC world do exactly that in order for one to see battery health.

Since the iPhone will tell you when the battery starts degrading (battery life remaining) there is some kind of measurement done by the phone. This could be simple by just counting charging cycles or more sophisticated by measuring mAh loaded as described above. If the latter is true and the phone software accounts for it then you might see a change in capacity after a few cycles. I doubt it however, why should Apple account for battery changes by third parties? If the capacity claim of the replacement battery manufacturer is correct, you most likely will just notice longer battery life.