How to monitor CPU core usage?
You can get a graph per core to show by going to Window on the menu bar and selecting Cpu Usage or Cpu History. There is also an option under the View menu to place a similar graph in Activity Monitor's dock icon.
If you're using a reasonably new Mac, Hyper Threading is enabled, so you'll see twice as many graphs as you have physical cores.
There are few ways to view the CPU usage in Activity monitor.
This one is the Dock
Those are in the Activity Monitor
You can also set it up as Floating window so it will be on top so you can see real time what is happening.