When do `initialize` blocks run?

Stream: new members

[Andy Phan]

I was reading about initialization in the reference manual (https://lean-lang.org/doc/reference/latest/////Elaboration-and-Compilation/#initialization). From the wording it seems like initialize would run before elaboration/compilation, but when I make an initialize block like below it also runs when I run the program after building it?

initialize
  println! "hello"

[Andy Phan]

Also, what does it look like when the initialize block is used to add an environment extension?

[Robin Arnez]

initialize blocks are evaluated when the module they're in is imported + when a program that depends on a module is run