This is a complex question。
Let start with something simple, Drupal 9.0 branch is identical to 8.9.x except for the Deprecated code removed。
Secondly, you need to go through your list of modules to see if it has version for Drupal 9.
Thirdly, you can't do an upgrade from older version of Drupal to Drupal 8 or 9, you must use the migration tool which will import your content from your older Drupal to Drupal 8 or 9.
This migration tool is genuinely nice, it gives you a chance to review and fix any issue before you do the actual migrate。
The Drupal 8.9.x will continue to have security support until November 2021。
In summary, the migration to Drupal 8 or 9 will be very time consuming for sure。