It depends on which airline you are flying with - I use this as my go-to for Pearson: https://www.torontopearson.com/Connecting.aspx##
For example, Air Canada uses Terminal 1 - bags go right through; WestJet uses Terminal 3 - have to pick up and recheck your bags
No, WestJet your bags will be automatically transferred as long as they are tagged right. No more need to pickup and recheck bags when connecting from Canada for travel to the USA. That system has been in place since before our last WDW trip in December 2015 -- they had just started then as I recall and were still in testing of the new system.