The Act joined Canada West, Canada East, Nova Scotia, and New Brunswick as “one Dominion under the name of Canada,” and it acted as Canada’s constitution until 1982. As Britannica details:
The British North America Act conferred on the new dominion a constitution “similar in principle to that of the United Kingdom.” The executive government was vested in Queen Victoria and her successors. These two provisions meant that Canada would have parliamentary and cabinet government. The legislature was to consist of a Senate, its members appointed for life from the regions of Canada, and a House of Commons elected from the provinces on the principle of representation by population. The act provided that criminal law should be federal and civil law provincial. The federal government was to appoint all senior judges, the provinces to administer the laws and maintain the courts. The act also authorized establishment of a Supreme Court of Canada.
The act also created the provinces of Ontario (from Canada West) and Quebec (from Canada East). A great deal of autonomy was granted to the Francophone community in Quebec to ensure the preservation of its traditions and way of life.
via Britannica Blog.