How to prevent apps from running in background in Miui 11 ??

2 non-root methods both ADB based.

Super-easy way: With ADB Wifi enabled (just need to connect to any Wifi AP be it a smartphone or router, even internet connection is not required ) you can leverage the power of Shizuku and freeze/defrost apps by using the Hail app (FOSS, available on F-droid).

Otherwise ADB.

Payzapp won't run as long as Shizuku is installed. 

Drawback :ADB permissions last only as long as the device is not rebooted.

You can also leverage ADB/Wifi to see the CPU usage per-app. Termux -> "top" command. Helps to ferret out rogue apps. 

Emui or miui. Problems are same with both.

Just force stop each app personally to get results 

i use app called ice box, doesn't require root

Another extremely easy way is to move less used apps to your Work Profile. The Island app can freeze apps without ADB permissions nevermind root. However this can increase RAM usage as another copy of  Play Store is created. It's only necessary to install and update apps so it can be freezed as well freeing up some RAM.

Step 1 - delete miui

1. Restrict Background Activity:
   • Go to "Settings" on your device.
   • Scroll down and select "Apps."
   • Find the app you want to restrict from running in the background.
   • Tap on the app, then select "Battery & Data usage."
   • Here, you may find options like "Background activity" or "Background power consumption." Turn off or restrict background activity for the app.
2. Manage App Permissions:
   • In the same app settings menu, check for permissions related to background activity, data usage, and battery optimization.
   • You can revoke unnecessary permissions that allow the app to run in the background.
3. Manage Autostart:
   • Go back to the main "Apps" settings menu.
   • Tap on "Manage apps' battery usage."
   • Look for the app in the list and tap on it.
   • Disable the "Autostart" option for the app. This prevents it from automatically starting up in the background when your phone boots or when you open other apps.
4. Clear Cached Data:
   • Still within the app settings, select "Storage & cache."
   • Tap on "Clear cache" to remove any cached data that the app may have accumulated, which can sometimes consume resources in the background.
5. Monitor Background Activity:
   • Keep an eye on apps that consume resources in the background. You can check this in the device's battery usage or data usage settings.
   • If you notice any apps consuming too much battery or data in the background, consider restricting their background activity further.
6. Third-Party Apps:
   • Consider using third-party apps from the Play Store that offer more advanced management features for controlling background activity and resource usage. Apps like Greenify or Brevent can help you further optimize app behavior.

These steps should help you restrict apps from running in the background on your Redmi Note 5 Pro without the need for rooting your device. However, please note that some system apps or essential services may still run in the background to ensure the proper functioning of your device. Be cautious when restricting background activity for such apps to avoid unintended consequences.

Same problem in my OnePlus Nord 3 CE - 12 GB Ram variant