FL 35
commercial jet pilot is descending from 35,000 feet , you asked.
Where I live , Im far away from CYYZ , tho there is a FIR hand off in my local town , at 135.400 Toronto Center is contacted , he will follow his flight ( IFR ) Plan , plus what other procedures when he is at 10,000 and below IE : Noise Abatement , etc.
Short Answer : He will be handed off to the next " Center" and follow his IFR flight plan.