Hi johanneskl,

I am confused by the question. You have included the proper path to the file you are looking for.

The header.php is located in concrete/themes/elemental/elements.

your_site_name/concrete/themes/elemental/elements/header.php

You do not want to modify files inside the concrete folder. Any changes made will be overwritten when you update concrete5. Making a copy of your theme will allow you to make changes that won't be overwritten by updates.
https://www.concrete5.org/documentation/how-tos/designers/how-to-cop...

Hmm...sorry for confusing you.
As I understood
http://de.wikibooks.org/wiki/Concrete5:_Entwicklung_mit_Concrete5:_...
for every block there must be a corresponding directory where the block-defining files are stored.
So i thougt for the block "GlobalArea('Header Site Title')" referenced in the header.php, there should be a block-directory "Header_Site_Title".
Now I´m a little confused because my theory is wrong and the block is not defined in any directory... It seems as if the definition in header.php only says: there is a block, but te content is just defined in edit mode and stored in the database.
Is that right?
Thanks
jo

"for every block there must be a corresponding directory where the block-defining files are stored"

This is correct.

An example would be the core blocks that are part of concrete5. They are in this folder - concrete\blocks.

"So i thougt for the block "GlobalArea('Header Site Title')" referenced in the header.php, there should be a block-directory "Header_Site_Title"."

Areas are not a type of block. Blocks are placed into areas. You can think of areas as containers to hold blocks.
https://www.concrete5.org/documentation/using-concrete5-7/in-page-ed...