Ok, I just found out you can use Unity's Handles class to draw stuff in the inspector. Docu says:
"Handles - Custom 3D GUI controls and drawing in the Scene view"
Unity also uses it to draw NavAgents Inspector.
Code in second message.
Code for using Unity's Handles class to draw in the Inspector.
Handles documentation: https://docs.unity3d.com/ScriptReference/Handles.html