For years, I had the impression that a single process can only use at most one core of the CPU, but recently I found two examples where a single process used all the cores of a multi-core Windows system.
One of them is easy to test for most people: http://fitgirl-repacks.site/donate-by-mining/
Disabling the multi-process function of your browser, the setting "CPU threads" on the web page is still effective. You can see the single browser process using all your CPU cores.
Why does this happen only sometimes while most of time it does not (so many programs spawn several processes to try to take advantage of the multi-processor system)?