If someone from Europe,(non EU country)gets an American visa, can they travel to Canada with that visa as well

No - Canada, as with any country, has its own laws concerning the issuing of visas. A visa issued by the US is based on US policy. The individual traveling should call a local Canadian Consulate or the Embassy and make an inquiry.

NO, Canada is a country of it's own with it's own immigration laws. Even when you usually would been able to travel on the visa waiver program, depending on what US visa you are applying for, you won't be able to leave the US until your green card has been issued and you didn't get the restriction stamp.

No you can't use an American visa to travel to Canada. you need to apply for a Canadian visa before you can travel to Canada.