Short answers, pulled from the story.
Jonathan Tweet, Monte Cook, and Skip Williams designed the d20 System for the third edition of Dungeons and Dragons. Jonathan Tweet helped forge the unified resolution mechanic alongside Monte Cook and Skip Williams.
Wizards of the Coast published the System Reference Document under the Open Game License in the year 2000. This release occurred before the third edition of Dungeons and Dragons was even released to the public.
The core mechanic requires players to roll a 20-sided die and add modifiers based on six character attributes. If the total equals or exceeds a target number known as the Difficulty Class, the action succeeds.
The boom ended in August 2003 when Wizards of the Coast released an updated version of Dungeons and Dragons, edition 3.5, at Gen Con 36. Third-party publishers received little warning regarding the update, leaving many companies with books that were out-of-date before they could even reach their audience.
The d20 System Trademark License was revoked with the release of the fourth edition of Dungeons and Dragons in 2008. This new license, known as the Game System License, replaced the original d20STL but contained no associated Open Game License or Open Content.