Well, select a scenario map from the map selector and play it. Sorry, what am I missing?

Indeed pathfinding may be one of the main issues of bridges - walking on objects could be done with fields already (as they represent buildable objects) but finding a single entrance and exit while actually walking on water (denied area) may not be so simple for pathfinding. Again - in settlers 4 or 5, I remember you could build bridges only at pre-dertmined sites that neeeded definition in the map itself (defined entrance and exit points). Actually, should it be possible to build a bridge everwhere you want? This might be an interesting feature like blocking ships from entering or leaving a river or even denying access to a beach (if building a U-shape bridge)?. A bonus of freely positioned bridges could be that they could be possibly built on land as well (zero height above terrain) and would then be called "roads".   I guess that would be a completely different game then.

Thing is, I was never able to play a scenario - how is one supposed to do ? When I choose a scenario in the maps, nothing triggers.

So it should be a parameter somewhere, in order to make it work for everyone (those who like you want to control finely the repair actions, and those who'd prefer it to be more automatic).

You already have that option, and it's on by default when you install the game.