How to make field painter like cities skylines districts?

Hello!

Can you explain me how to achieve such an effect as did the developers of Cities Skylines in feature called districts?

I mean... How to draw those fields with boundaries updated at runtime, edit it and substract by creating other near it.

I know they were using Unity to make this game, so i it has to be possible to achive, but i have no idea how - I don't even know how to draw a polygon.

I know this topic - I learned a way explaining it a little bit, but I still don't know how to start.

So i broke the problem into smaller segments (which are my questions):

1. I need to know how to draw a (round) polygon at runtime.

2. I need to know how to use knowledge from here in practice and combine the new polygon with older one.

3. I need to know how to animate the boundaries.

And i think it's all. Any answers and comments are appreciated. Thanks in advance!

EDIT: How about the navigation system in unity and baked ground with non static objects moving around? You can move this object and the Navmesh is updating quickly. How is that done?