block

Facts

Drupal core ships Block and Layout Builder modules.

• Block module is in business of placing blocks.
• Layout Builder module is in business of placing blocks.

But...

Block placements are decentralized: Each block placement is a config entity. Several modules can work together to place blocks on the same page. In other words, a module providing a block placement, doesn't know if a different module is also providing a placement on the same page or even on the same region.