5950X Core Utilisation

Quick Q, if a program is using 8 cores will windows make use of the unused cores for background processes or will it always try and use core 0 even under heavy load?

In HWInfo are you able to see cpu utilisation for cores 0-7, for example, opposed to an average of all 16 as if a program uses 4/6/8 cores 100% but the remaining 8 are semi idle, the usage would display at 50-80% despite the active cores being at 100%.

Thanks