where is my apps library since iTunes upgrade

Since you do not have App Store in Mac iTunes there's no as before using iPhone/iPad/iPod touch to view apps

To check for updates:

Your device's App Store app > Updates (bottom right)

All the apps you've purchased but not on your device:

Your device's App Store app > Updates (bottom right) > Purchased > select the app you want and tap the cloud symbol to download onto your device (not sync)

Press and hold app's icon till it shakes and then tap the X to delete from device.

The library itself is where it has always been; ~Library>Music>iTunes>Mobile Apps.

You can delete it and save some room on your HDD. Or back it up to an external drive and then delete it from your HDD. iTunes can no longer use it.

Late 2012 Mac mini, macOS 10.12.6;  Watch, watchOS 3.2.3; iPad Air 2 & iPhone 6+, iOS 10.3.3; TV 4th Gen, tvOS 10.2.1;  Airport Express

The new iTunes 12.7 do not have App Store or library.

You download/update/purchase apps directly using the device.

As said it can be done on the device.