Firstly, make sure you have ssh access to the server (dev, test and/or prod). Your ssh command looks something like this: ssh email@example.com or ssh firstname.lastname@example.org. Drush alias works by using this username and domain/ip address combination to login to the server and execute the command and then return back to your command line. In your project root, create a drush folder and inside it, create a sites folder. Now you put your site alias config in that folder.
All you have to do is intercept every request, check if it is authenticated user and if not redirect to login page. There is an Event that is fired on every request, we need to subscribe to that event.
This website https://abh.ai has been hosted on almost all of the major players in the cloud hosting business. I started out with Scaleway around 7 years ago when they were providing these small ARM options with 20GB SSD. It was the cheapest solution back then costing less than 5€. But that doesn't mean it wasn't a good solution. It was excellent! It was fast and reliable. But because it was ARM and at the time a 32-bit processor, many things like docker could not be properly installed. This is why I decided to look for alternatives, which eventually brought me back to Scaleway!
Upgrading a Drupal 8 site to Drupal 9. Follow the blog written by Dries on Drupal.org: https://www.drupal.org/docs/upgrading-drupal/upgrading-from-drupal-8-to-drupal-9-or-higher to easily upgrade.