How to: Project Architecture for Unity3D or Spaghetti code?

For more than 2 decades that I am writting code (several programing languages) and in all I had always used design patterns and several kinds of architertures..... Layers, separation of responsabilities, different class libraries, clients / servers, etc...

However, during this weekend just for fun and for curiosity I have installed Unity 3D and gave it a shot. Although I was able to open the files in Visual Studio, I did not understand the default file structure of the unity project.

I created nested folders in order to organize my code, but as soon as I was trying to create a class many questions arised:

• Where is the namespace? Why do I need to write them myself? Shouln'd they be automatic and taking in consideration the "folders" structure?
• How can I create/reuse/import different project class libraries?
• how to manage nuget packages?
• and everything that I have been doing all my life in programming.....

I found an article with some info, but that is it... nothing else.

I was wondering if someone could point me to the right direction regarding this subject.

Thanks.