My iPhone 16 Overheats & Power Button is Broken

I am absolutely furious with my iPhone 16 256GB that I bought from Media Markt! I deeply regret this purchase. The phone's screen lock/power button is completely erratic: sometimes it makes no sound, and other times it makes a sudden, loud "thump." While it was fine initially and I had no other phone to use, it has progressively worsened.

Now, I can't even access the iCloud payment and delivery section; it throws a "no connection" error, even though both my home Wi-Fi and phone's mobile data are working perfectly fine. I upgraded from an iPhone 11, which had zero issues. Another major problem is its severe overheating. Especially recently, it gives overheating warnings even when it's not physically hot. Specifically, when charging, the phone is cold to the touch, but it still warns of high temperatures.

Paying this much money for such a problematic device is pure regret. If I take this to the store, they'll just send it to service, and I don't have a spare phone. What kind of service is needed for a phone that's only three months old? Apple and Media Markt, you need to fix my iPhone 16 immediately! Resolve the power button issue, the iCloud connection problem, and the constant overheating. This is unacceptable for a brand-new, expensive phone.