I am going from Amsterdam to Miami via Toronto. Do bags go thru to MIA?
Or do I also do customs in Canada ?
Here's the relevant airport page:
International flight to U.S. flight
Canada Border Services Agency*** & U.S. Customs and Border Protection
*** Some International to U.S. connecting flights at Terminal 1 do not require passengers to clear CBSA before clearing U.S. CBP inspection.
So, you will do cross the US border in Toronto but whether you need to pick up your luggage or not depends on the airline(s) involved. If you'd disclose the airline(s) involved, we could perhaps give you more advice. In general, for the future: you can't give too much information. Don't put personally identifiable information in the question but include everything, even if you think it can't possibly be relevant. This includes which country issued the passport (or passports!) you hold, additional travel documents like NEXUS, the date of your flight and the airlines you are flying. If you have flight numbers, even better. Yes, there have been cases when any of these affected the outcome of similar questions (dates can be relevant for example when construction or new terminal openings affect operations not to mention when they move the entire airport like they did with Hong Kong twenty years ago or Istanbul two weeks ago).