The example from silverstripe.org/learn/lessons/introduction-to-the-orm?ref=hub, I think I understand the $LatestArticles function part. But where do I define the creation of the records like in the first part of the lesson where $product = Product::create(); Creation goes to controller or where? I'm trying to populate tables coming from a large dataset (from textfile) into defined tables in SIlverstripe.

There are two good places for this. The BuildTask class or the DataObject->requireDefaultRecords() method.

The BuildTask is a simple task. You can find them under "mysite.com/dev/tasks"

<?php
class MyDataObjectTask extends BuildTask {

	protected $title = 'MyDataObject Build Task';
	protected $enabled = true;
	
	public function run($request) {
		$list = array(); // dataset to import
		foreach ($list as $item) {
			$myDataObject = MyDataObject::create();
			$myDataObject->Foo = 'Bar';
			$myDataObject->write();
			DB::alteration_message('MyDataObject #' . $myDataObject->ID .' created', 'created');
		}
	}

}

DataObject->requireDefaultRecords() is a method in your DataObject. It will be automatically called on /dev/build.

<?php
class MyDataObject extends DataObject {

	private static $db = [
		'Foo' => 'Varchar(3)'
	];
	
	public function requireDefaultRecords() {
		parent::requireDefaultRecords();

		// execute only if MyDataObject is empty
		if (!MyDataObject::get()->exists()) {
			$list = array(); // my dataset to import
			foreach ($list as $item) {
				$myDataObject = MyDataObject::create();
				$myDataObject->Foo = 'Bar';
				$myDataObject->write();
				DB::alteration_message('MyDataObject #' . $myDataObject->ID .' created', 'created');
			}
		}	
	}

}