Prefix of drupal: in front of certain dependencies and what does it mean? I saw certain modules list their dependencies with a prefix of drupal:, while other don't.

What's the difference between:

name: Core
type: module
core: 8.x
dependencies:
  - block


And

name: Core
type: module
core: 8.x
dependencies:
  - drupal:block


Notice the extra prefix drupal: in front of block.
The 2nd is the correct way as per: https://www.drupal.org/docs/8/creating-custom-modules/let-drupal-8-know-about-your-module-with-an-infoyml-file but I think drupal is assumed for the project if its not specified (The first part before the : )

So for Drupal core modules they'd prefix with drupal: and for custom modules its usually the module name e.g. webform:webform as they are projects of themselves