doesnt windows almost always use more then one core, program and background tasks
Yeah its core balancing background tasks, stuff with low priority gets moved around to make room for high priority on less occupied threads, so there is never just one thread being used while the rest are idle.
This is not just Windows 10, so the whole maximum Turbo thing is a nonsense. If the system if doing something you're lucky to see even multi-threaded Turbo working.