All questions are highly related, so I preferred to ask them together.

Q1: Why is the electric field uniform between parallel plates and is it only uniform between? and not outside?

Q2: Why uniform? Which rule exactly points/proves this out? Putting test charge in any place would feel the same force? how? What If I put it positive test charge closer to negative plate - shouldn't the force be stronger? The rule of uniform charge distribution doesn't prove it to me.

Q3: one of the rules for this is to be far from edge. Does it imply to have infinite length parallel plates? What practical use case would this have if we only have the system defined for infinite plate?