What causes iPhone 7 overheating and cellular outage issue?

Hello all,

I've an iPhone 7 (iOS 15.5). This device had problems with overheating and fast draining. Even when I didn't operate any application, it became almost unbearably hot. When I took it to the repair store, he replaced the battery and returned the phone to me because the battery level was 67%. But the problem persisted. After replacing the battery, the cellular connection dropped. Especially when I talk to someone on the phone, the connection drops. Each time, I turned the phone off and on again. When I went back to the repair store, he said that the unit draws 23 amps of current, that there might be a short somewhere on the motherboard, and that I need to leave it for a while. He'll keep it in a special solution for a while and clean it because of a possible liquid contact. Later he explained that he replaced the chip because there was a malfunction in the cellular chip. In this way, he delivered the device again. But the overheating of the phone, fast draining and the interruptions of the mobile network continue, what could be the problem of this phone?

Overheating problem: It's most noticeable in the area you see in the picture just below the camera.