Short answers, pulled from the story.
Dungeons & Dragons retro-clones are games that emulate earlier editions of the original game no longer supported by the publisher. These games deliberately avoid using the brand names Dungeons & Dragons, D&D, or any other trademarks associated with the franchise while preserving the soul of the original experience.
The System Reference Document was released in 2006 by Wizards of the Coast under the terms of the Open Game License. This legal framework permitted the use of specific game mechanics and terminology without infringing on trademarks, enabling the creation of Dungeons & Dragons retro-clones.
Matthew Finch was the initial author of OSRIC, which was released to the public in 2006 as a retro-clone of the first edition of Advanced Dungeons & Dragons. Stuart Marshall later took up the project and released it to the public in 2006.
Labyrinth Lord extends the rules so characters can advance to 20th level, whereas the 1981 Basic Set only included levels up to 14. In addition, clerics receive spells at first level in Labyrinth Lord, a nod to Advanced Dungeons & Dragons.
The Core Rules 1st Edition of Basic Fantasy was released on the 17th of February 2007. The game was updated in 2008, 2014, and 2023, and has been positively received for its accessibility and faithfulness to the original vision.
Old-School Essentials is a clone of the 1981 Dungeons & Dragons Basic and Expert sets, edited by Tom Moldvay and David Zeb Cook respectively. As of 2020, it comes in two versions: Classic Fantasy and Advanced Fantasy.