What is the difference between static linking with .a and .so libphobos?

[WhatMeWorry via Digitalmars-d-learn]

I'm studying the D article at:

https://dlang.org/articles/dll-linux.html#dso5
https://dlang.org/articles/dll-linux.html#dso6

Statically Linking D Program With libphobos2.a
Statically Linking D Program With libphobos2.so

Don't we want to Dynamically link with shared libphobos2.so?

Also why doesn't Windows have a libphobos.dll file. I only find a 
libphobos.lib