There's actually a
Connecting Guide which asks you on the YYZ airport site where you're coming from (Canada, US, International) and where you're going onwards from Toronto (e.g. US, Canada, International) along with the airline for each. It then gives instructions.
So, say, arriving in Toronto from the US on United, and then going onward within Canada on Air Canada, it clearly states that a baggage pickup/re-check/re-screen is required. Even with those same options but both flights (incoming to toronto/departing) on Air Canada, a pickup/recheck/re-screen is still required.