I am experiencing battery drain and heating issues even when my iPhone is idle

After an update, certain related tasks including indexing may continue in the background, in which you may experience an increased battery drain and thermal temperature. Home and Lock Screen significant battery consumption plus an Insight is usually an indication that these tasks are still running. When the update is fully complete, your iPhone will return to normal conditions.

I suggest you read my User Tip regarding this: Addressing the Common Increased Battery Drain and Thermal Temperature Conditions Your iPhone May Face After Setup and Updates

Hello~ There’s a message that your update is finishing in the background….so your iPhone is in fact “doing something” I would take a look at that update.

If your iPhone or iPad won't update - Apple Support

~Katana-San~

If your iPhone is experiencing battery drain and heating issues even when idle, it could be due to several factors including: background app activity, location services constantly running, outdated software, faulty hardware, excessive notifications, or a bug in the iOS operating system; check your settings to manage background app refresh, location services, and notifications, update your iOS to the latest version, and if the issue persists, consider contacting Apple support to diagnose potential hardware problems. 

Here are a few steps to troubleshoot:

• Check for active background apps:
  • Go to Settings > General > Background App Refresh and disable unnecessary apps. 
• Manage location services:
  • Access Settings > Privacy > Location Services and review which apps are allowed to access your location, limiting it to only necessary apps. 
• Review notification settings:
  • Check your notification settings for apps that may be sending excessive notifications, which can drain battery. 
• Turn off unnecessary features:
  • Disable features like "Hey Siri" or "Raise to Wake" if you're not actively using them. 
• Check for faulty hardware:
  • If the issue persists even after adjusting settings, consider contacting Apple Support to check for potential hardware problems with your battery or other components. 
• Use Low Power Mode:
  • Enable Low Power Mode to temporarily conserve battery life by reducing certain features.