Your Battery drain depends on the Charge Cycles, Battery Health, and the apps that are using your battery. The phone has no idea if it has been paid for or is being financed.
For newer phones, the battery is expected to last 1000 charge cycles before it will need to be replaced, irregardless if you purchased it in full or financed it.
As for the claim that Apple got exposed for tampering with batteries, I don't think you have any idea on what that was about. The issue was that Apple throttled the CPU on devices with a degraded battery without letting the user know. Nothing was done to the battery on those devices it was the CPU, and that was back on the iPhone 6. The change is that there is now a message in Settings that says your device is being throttled due to a degraded battery. The throttling of the CPU is still done today on a degraded battery to prevent the device from crashing and overheating. And you will still need to replace that battery to return the device to optimum performance once the Battery Health drops below 80%.
Are you looking for assistance with finding the source of your battery drain and the reason why it is happening?