Is it possible to install Drush 9 without using Composer? Is it somehow possible to install Drush 9 without using Composer?
No.
As it says in the docs:

Drush 9 only supports one install method. It requires that your Drupal 8 site be built with Composer and Drush be listed as a dependency.

If you don't know how to use composer, you need to learn now, because you'll have serious problems using Drupal without it.