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 ?