Will we need to clear customs in Canada if it is just a layover before moving on to Amsterdam?
Where is the origin of the flight that lands in Canada? All one ticket ?
Also ... through which airport in Canada are you connecting?
Yes, all one ticket from the Us to Canada to The Netherlands
Connecting through Toronto
When we did this a few years ago in Toronto, there was signage for a special direction to go through a passport control (immigration) checkpoint for transiting passengers. Not Customs which is for baggage. Airport configurations can change.
If you fly home through Toronto, you will go through advance US immigration procedures (not customs) in Pearson airport. Just be sure to go to the correct arrivals area, not the big crowded room for those who are staying in Canada. The direction signs are fairly clear and the plentiful staff are used to directing foot traffic.