What to do when a car turns into a side road (etiquette)?
As you're in Ontario the following references are official.

Look at the picture at the bottom right of Toronto's Understanding Bicycle Lanes -- here's an excerpt:



In summary, stop behind or pass to the left of the turning car.

I generally expect drivers to see what's happening out the front of the cars, but never expect them to know what's happening to the side of their cars.

In the illustration above, for example, I guess the driver is looking forward (at the intersection which they're turning into), not looking sideways (at the space between them and the sidewalk where it says "No").

Ditto at stop signs and stop lights, by the way. Especially if it's a truck, beware of pulling up close to the intersection squeezed between the truck and the sidewalk: because if the truck turns right without seeing you're there you'll be squeezed between the truck and the street furniture; so hang back in the bike lane away from the intersection (give them space to turn if they're going to), or even temporarily take the lane (occupy the car lane) behind them until traffic starts moving again.

See also this page, Car Bike Collisions | Motorist Right Turn, which says,


  Motorist turning right collides with cyclist travelling in parallel direction
  
  Key problems:
  
  
  motorist overtakes cyclist just before turning right
  cyclist tries to pass to the right of right-turning motorist
  cyclist riding on the sidewalk fails to stop and yield to turning vehicles at intersection
  
  
  To avoid:
  
  
  look ahead for turn signals
  watch out for drivers slowing down in preparation for a right turn
  stay away from the driver's "blind spot" (near the right rear wheel)
  do not pass right-turning drivers on the right
  use the road, not the sidewalk, to be more noticeable to motorists
  




What if cyclists are behind me; what if there aren't?

On a ski slope it's the job of people up-hill to stay out of the way of people down-hill. I reckon the rule's similar for cycling in traffic, not to mention driving, i.e. do what you need to and count on cyclists behind you (if there are any) to not run into you (I don't know, maybe that depends on where you are and how fast you're going).

But use hand signals when you change lane, and ideally also when you slow i.e. when you're about the brake, that's what you can do for them.