OSX Hierarchy questions and organization

I've done extensive searching on these questions and have even come across others, however, they do not answer any of my questions.

• What are ~/Library and /Library for respectively? I can see they store certain applications, but why there and not /Applications or ~/Applications? I'm confused between the 4.


• In my personal system, I see go, git, and android-sdk directories in /usr/local for these programs. Why would these not be stored in those application or library directories I mentioned above?


• I know this is all down to choice, but where do I store, or what is a common place to store libraries such as boost that I would use across all of my C++ programs?


• in line with the question above, where should I store the projects/source files for my C++, Java, IOS, Android, etc. projects? I feel like my system right now is very unorganized with some projects on my desktop, home directory, etc. because of my ignorance on these things, I want to find a good, organized system.