What is causing unknown network traffic on an Android phone? I have a Xiaomi Redmi Note 8 Pro smartphone with Android 10 installed. Even when all apps are closed using the task manager, I see some realtime network traffic. It's not that much but it goes on all the time. This is also the case when not connected to Wifi (using 3G/4G).
Is there any way to check what specific apps/processes are consuming data?
Before identifying and potentially vilifying specific apps, it is important to know that any app can schedule deferred background operations with the system.  This sort of background work can and often does include network requests, depending on the app.
My recommendation is to familiarize yourself with your system.  For example, inspect your account for any syncing settings.  A plethora of Google apps like to keep themselves up to date through Google Play Services.  I suspect that Redmi has OEM apps, or the OS itself, contributing to passive network activity as well.
If you would like to inspect which apps are sending packets, I suggest looking into network inspection tools such as those built into your router, or with programs like Charles Proxy.