Short answers, pulled from the story.
Sega was officially founded on the 3rd of June 1960, when Martin Bromley established two successor companies to Service Games of Japan. The company traces its roots to Standard Games, which Bromley, Irving Bromberg, and James Humpert formed in Honolulu, Hawaii in May 1940.
Sega's first self-manufactured arcade game was Periscope, a submarine simulator released worldwide in the late 1960s. It featured light and sound effects that were considered innovative at the time and helped establish the 25-cent-per-play pricing standard for arcades in the United States.
Sonic the Hedgehog, released in 1991 as a bundled game with the Genesis, drove the console to outsell the Super Nintendo nearly two to one in the United States during the 1991 holiday season. By January 1992, Sega controlled 65 percent of the 16-bit console market in the US.
Sega discontinued the Dreamcast on the 31st of January 2001, and restructured as a third-party software developer after three consecutive annual losses totaling billions of yen. Competition from Sony's PlayStation 2, Nintendo, and Microsoft's upcoming Xbox made it impossible to sustain the hardware business.
Sammy Corporation acquired Sega in 2004 in a stock swap valuing Sega between $1.45 billion and $1.8 billion, forming Sega Sammy Holdings. Sega transitioned into a third-party publisher and acquired studios including Creative Assembly, Sports Interactive, Atlus, and Rovio Entertainment.
AEGIS, the Allied Employees Guild Improving Sega, is a labor union formed by Sega of America employees affiliated with the Communication Workers of America. Workers voted 91-26 to form the union on the 10th of July 2023, making it the first video game industry union of its kind in the United States.