What i had to look up to port a Drupal 7 module to Drupal 8

Some of it, anyway.

This is… sort of wrong:

https://drupal.stackexchange.com/questions/186156/drupal-8-variable-get#186160

https://www.drupal.org/docs/8/converting-drupal-7-modules-to-drupal-8/d7-to-d8-upgrade-tutorial-convert-hook_menu-and-hook

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Url.php/class/Url/8.6.x

How to return a renderable array (Drupal won’t accept a plain string anymore) in a controller to display a page:

$build['#title'] = 'Page not found';
$build['#markup'] = 'The requested page could not be found.';
return $build;

From https://cgit.drupalcode.org/search404/tree/src/Controller/Search404Controller.php?id=3683c2e

https://stackoverflow.com/questions/50862690/drupal-8-translatable-configuration

https://www.drupal.org/docs/8/creating-custom-modules/defining-and-using-your-own-configuration-in-drupal-8

https://www.drupal.org/docs/8/multilingual/translating-configuration

https://www.drupal.org/docs/8/api/configuration-api/configuration-schemametadata