What exactly is the airspace above CYYZ (Toronto)?
Canadian Class C airspace is generally the same as US Class C airspace. Prior permission is required before entering. Canadian Class B airspace is 12,500' and above. Not a factor for you most likely.

I recommend reading the VFR legend to better understand the markings. It's all there if you purchased a map. The 125/25 marking is the ceiling and floor of the airspace. What you're looking at is the US Sectional, not the Canadian one however. The square [25] refers to the control zone around the airport. You won't be landing in Pearson, so you definitely won't be in their control zone. It extends about 8nm from the airport from surface to 2500' and most busy airports have one. All control zones are class C airspace.

What you're looking for on the VTA is VFR planning. It will have recommended routes to follow, as well warnings and information needed to be able to plan a VFR flight through the area.