Many reserves do indeed have their own police forces internally. In fact, when I was in high school, a Fort Frances City police officer pulled over a First Nations police officer who had their emergency signals activated, for crossing a double line, on a provincial highway (just outside city limits) It was a shit show. My father was involved in mediating that, there was a real world consequence to that officer not being able to respond to that call, and the city officer admitted he was concerned with making a jurisdictional point (even though, even if he was technically right, he also ended up being technically wrong, the road had become provincial). In another instance, a first nations officer got into an accident responding to a call on the reserve with their emergency signals activated (ran a light iirc), at the time, they were within city limits, so the city decided to issue them a traffic violation and charge them.