Reading textarea values per row: what is the internal method? If I'd like to create a textarea field for a admin configuration form that would include different configurations on multiple rows then how can I read these values/rows?

I know there must be a internal method for this since it is very common form field/input type. I just wouldn't like to invent it again.

And I just ran into a module that was using this same principal but it matched the rows with regex \n\r which turned into few gray hairs if you had only one value since it didn't then match if you didn't deliberately press enter. And for clarity it obviously didn't check that if the first line included text, just that regex and therefore no settings were saved.

That was also a wake up for that there must be some more intelligent and elegant way...
Drupal doesn't provide a method for that.

Just use whatever PHP splitting method makes sense for your use case. A very simple example might be:

$text = str_replace(array("\r\n", "\r"), "\n", $text);
$vals = explode("\n", $text);