If there is not, how long does it take to clear customs and immigration when arriving from US?

There are kiosks. But just for reference (and for future readers), there is just one general immigration line at Billy Bishop, i.e. no separate line for Canadians. But it generally moves pretty fast, as the vast majority of passengers are either American or Canadian (the only international flights out of YTZ are to the US).

Also, how do you get from the plane to the terminal: bus, walk, other?

You walk. It's a small airport. From the terminal to the Toronto mainland, you can walk through the underground tunnel or take the ferry.