Using ddev I created a webserver locally and I have a Drupal 8 project with drush 10, but I need drush 9. How can I change the version of drush in the project? Thanks so much
As @leymannx says in the comments, this isn't really a ddev question, it's a composer question, but it's easy.
ddev composer require "drush/drush:^9"
will get the most recent Drush 9 version.
But I should mention that it's unlikely that you actually need Drush 9, Drush 10 is what most people are using with Drupal 8 and 9.