My iPhone 13 Pro 120hz is not working

Since you bought a used iPhone it certainly could have had the screen replaced. If you go to Settings > General > About, do you see anything which says unauthorized parts are installed on your phone? You can take it to an Apple Store and see if they would be willing to replace the screen with an authentic screen, but they may refuse to do it. Apple typically won't touch an iPhone which has been tampered with.

Are you comparing the motion apples to apples, as in on the exactly the same pages or sites? Ultimately, since there is no setting, which shows whether you are on 120hz or not, if you are convinced there is an issue, you should schedule an appointment at your Apple Store Genius Bar and let Apple see the phone. Since it's an iPhone 13 Pro, which was discontinued in September 2022, your phone is most certainly no longer under warranty. If a repair is required, you'll have to pay for it. I guess the question is how important is this to you? Another question would be if the phone has ever had the screen replaced? And if it was, was it replaced by Apple itself or an Apple Authorized Service Provider, like BestBuy in the US? If it was replaced by anyone other than Apple or an Authorized provider, you likely don't have a Promotion Display on the phone. Only Apple or an Authorized Provider have access to authentic Apple Parts. If this is your situation, there would be no point in taking it to Apple. They won't touch a phone with aftermarket parts installed.

Finally, did you buy the phone brand new from Apple when they still sold them? Or perhaps, did you buy a used iPhone, which could be refurbished, not by Apple.

satgun wrote:

it does not say anything about unauthorised parts

Then I recommend you do as I suggested earlier and schedule an appointment at your Apple Store Genius Bar and let Apple see the phone.

That's of course up to you. But you need to know Apple discontinued the iPhone 14 Pro models in September 2023. It would be HUGELY unlikely anyone can sell you a brand new 14 Pro at this point. You may be able to buy one which has been refurbished, but that can be risky. If you are focused on an iPhone 14 Pro, since it's pretty much a guarantee it will be used, read this --> The All Too Common SAD Reality of Buying … - Apple Community

I'm curious to know how you know it is at 60hz if you have Limit Frame Rate turned off? There is, to my knowledge, nothing which tells you what the frame rate is when using your phone.