Getting into Preschool/Nursery School

I am looking for advice and anecdotes on what to expect in the preschool/nursery school enrolment process. We live in the GTA downtown (near U of T).

My daughter will turn 2 in April and we would like to get her into a preschool starting this October when she turns 2.5 (30 months, which as I understand it is the starting age for preschool?) or 3 years old at the latest.

Of all the places I have contacted so far to get on the waitlist, several have told me (or listed on their website) about the other families that get precedence — e.g., children of staff, children with older siblings enrolled, children transition from the daycare room, etc. None of these apply to us; she’s not in daycare right now.

So what happens if we don’t get off a waitlist? How common is it for children to not go to preschool? If I am understanding right, schooling is not mandatory until age 6 (??) but then that means we cannot be guaranteed a spot in our neighbourhood school until she turns that age??…

Can someone explain to me how this all works? I am a first time parent and new to Canada, just trying to figure all this out.