Gameplay tactics/differences between Open/Realistic

I understand the game differences between Open/Realistic (Fly to/from anywhere, Build Terminals anywhere and different gate limits)

But how does the actual gameplay change?

Is it more about branching out to different countries to find the perfect Route? Or do you still try to stay within the initial Country?

Open is about, as you said, finding the most profitable route anywhere in the world, while in realistic you have to remain within your starting country, with the exeption of Europe after 2000 due to the Open Skies Agreement.

How do you decide what route to create next? I know you can Add Route from the airplane where you can select the Airports you have Gates for, the list is then sorted roughly in PAX Demand.

Is there any other way?

There is no way of changing the sorting order, which I believe is by gate cost, and higher gate costs usually equal higher demand.