Android system takes too much data usage
Android System data usage

When you use tether your Smartphone and your PC the I assume that those data will be accumulated in the "Android System" entry - or did you encounter a separate entry in the Android network usage list?

On your PC you should check if the tethered network connection is marked as cellular / metered connection network connection (Windows). 

Background data user

You can restrict the background data usage for ever app separately. Therefore you can spare the apps where you want/need background data.