If you have to go for a Drupal core update from a previous install based on composer_manager or drush (both deprecated), consider installing something robust, Composer friendly, that excludes vendor and core for a lightweight repository and enables CI and automation tools like Phing, Behat, PHPUnit, CodeSniffer, ...
If you don't have Drupal VM installed, head to the documentation.
If you look for ideas to decorate the office, here are my two favorites from Drupal.org
The l() function is deprecated in Drupal 8, here is a starter.
Import the URL and Link classes :
You have several options, but basically, you can start with an url that will be used by a link.
One of the possible implementation will be used, with the Link::fromTextAndUrl method.