Where can I find the code how wordpress parses a post?

The specific function you are referring to is wpautop, which adds paragraphs & HTML linebreaks to already intermixed HTML & plain text.

wpautop is part of a family of content-processing functions hooked to the_content, a filter applied to the post content after pulling from the database just before outputting. Taking a look at wp-includes/default-filters.php, you can see the default functions are:

add_filter( 'the_content', 'wptexturize'                       );
add_filter( 'the_content', 'convert_smilies'                   );
add_filter( 'the_content', 'wpautop'                           );
add_filter( 'the_content', 'shortcode_unautop'                 );
add_filter( 'the_content', 'prepend_attachment'                );
add_filter( 'the_content', 'wp_make_content_images_responsive' );

You can remove them with remove_filter( 'the_content', 'name_of_function' ), and add your own with add_filter( 'the_content', 'my_function' ); – see the documentation for more information on adding filters.