A question which I see quite often in response to posts like A modern way to build and develop Drupal 8 sites, using Composer is: "I want to start using Composer... but my current Drupal 8 site wasn't built with Composer. Is there an easy way to convert my codebase to use Composer?"
Unfortunately, the answer to that is a little complicated. The problem is the switch to managing your codebase with Composer is an all-or-nothing affair... there's no middle ground where you can manage a couple modules with Composer, and core with Drush, and something else with manual downloads. (Well, technically this is possible, but it would be immensely painful and error-prone, so don't try it!).