Is my iPhone heating because of iOS 26.4 or something else?

Had an iPhone 13 almost 3 years old, was at 80% battery health. Post update, kept heating up, I thought it is normal and ignored, few weeks later, battery is swollen, the screen is bulging outwards. Phone keeps turning off all the time.

Bought an iPhone 17 yesterday. Charging it, still heats this one too.
These are the charging options I tried:
- connected to wall socket with Apple Official 20W Charger
- connected to USB-C port on my Mac
- In Airplane mode
- without case

No matter the case, the phone still heats up, a lot!

This can't be normal. Never faced heating issues before. I seriously doubt the iOS update to be the culprit here. What do you guys think? What's your experience been?