What are nodes and how do you put them in a map?

What are nodes, how do they work (if you know) and how do you put them in a map?

They are entities that npcs use to navigate the map. You put them in your map by placing info_node entities in Hammer.

Note that you don’t need to place them randomly around the map, place them at corners and stuff.
For helicopters and gunships to work you need air nodes.

nodes in the air? or special air nodes

I think the air node entity is info_node_air