Initial steps on navigation meshes

Hello. I’ve been reading about navigation meshes because grid-based pathfind doesn’t suit the game I’m making because it creates unreal paths.

I know how to implement a simple A* algorithm for a grid base game with nodes, but for navigation meshes I’m a little lost.

Consider this map from my game with the following navigation meshes (they are done poorly, I know, but it’s just for the example):

How i use the polygons for the AI to walk?

Are usually just the center and vertices used for navigation and a straight line to go inside them?

Do you usually use center-to-center or how do you discern which point to use?

What if I want to modify the meshes with dynamic obstacles? How can I do that in real time?

Consider that I want to make the map and it’s meshes just once and at the beginning of each round place random objects through the map, so the fixed navigation mesh should now take into account those newly generated objects…