Who exactly is the "Premier of Canada". I read newspapers from the 1930s and 40s and they use that term?

Generally we say the "Prime Minister of Canada" or a "Premier of a province," but in fact the two terms are interchangeable. It's perfectly acceptable to say "premier of Canada" instead of "prime minister of Canada." Likewise, it's acceptable to say "prime minister of Quebec" instead of "premier of Quebec." In French, they only have one word: "premier ministre"