Below, and in the column at the right are a few links I've used for genealogy searches in the locations that are now within the present day Province of Ontario, Canada.
Prior to the close of the American Revolutionary War (1783) the land area that is now the Province of Ontario was within what was known as the Province of Quebec, a part of the British Empire in North America. The other British provinces in North America were Newfoundland, Rupert's Land, Nova Scotia, Prince Edward Island, the thirteen Colonies that would become the United States, and East and West Florida. In 1783, by the Treaty of Paris, the British would lose the thirteen U. S. Colonies, who gained their independence, and East & West Florida, which were returned to Spain.
In 1791, the British established the Province of Upper Canada and the Province of Lower Canada. The prefixes, Upper and Lower, were given to the two new provinces based upon their orientation with respect to the St. Lawrence River which flows from the eastern edge of Lake Ontario, east and north, to the Gulf of St. Lawrence. "Upper" Canada is near the headwaters of the St. Lawrence River, or "up" river, while "Lower" Canada is "down" river from the headwaters. The Province of Upper Canada comprised much of present day southern Ontario. It bordered New York State and the northern shores of the Great Lakes. It was created to accept Loyalist refugees from the new United States. The Province of Upper Canada existed until February 10, 1841.
In 1840 the British Parliament merged the Provinces of Upper Canada and Lower Canada into the Province of Canada. The western portion of the new Province of Canada, being the area that was formerly Upper Canada, was known as Canada West. It was a separate political entity, had its own seats in the Assembly, and existed until July 1, 1867 when the Dominion of Canada was created with four provinces, Ontario, Quebec, New Brunswick and Nova Scotia.
As mentioned, Upper Canada was largely populated by Loyalist refugees from the American Revolution. Many of these Upper Canada pioneer families later had descendants who returned to the United States, many of whom settled in "upstate" New York, Michigan and Wisconsin. Reconstructing a line of ancestry that included a Loyalist family can often involve researching a generation or two (or more) in present day Ontario, Canada.
Archives of Ontario
Kawartha Branch, Ontario Genealogical Society
Ontario and Upper Canada Genealogy and History
Ontario Land Records at Olive Tree
Ontario Vital Statistics Project
Quinte Branch, Ontario Genealogical Society
Upper Canada Land Petition Records
West Lake Quaker Minute Books and Registers (Prince Edward County) Some Adolphustown Meeting records too.