Short answers, pulled from the story.
Sega traces its origins to the 3rd of June 1960 when Martin Bromley and Richard Stewart established a new company in Tokyo. This entity evolved from Service Games of Japan, which itself began as Standard Games formed in May 1940 by Bromley, Irving Bromberg, and James Humpert.
The merger occurred in 1965 when Nihon Goraku Bussan acquired Rosen Enterprises led by David Rosen. The combined entity became known as Sega Enterprises Ltd with Rosen serving as CEO and Stewart named president.
The SG-1000 sold 160,000 units in its first year far exceeding projections of 50,000. Despite this success it was quickly outpaced by Nintendo's Famicom due to hesitancy in collaborating with third-party developers.
The surprise early release in North America on the 2nd of September 1995 alienated retailers who were not included in the rollout. KB Toys refused to stock Sega products while Sony's PlayStation launched just two days later and surpassed Saturn sales within that same timeframe.
Sega announced the discontinuation of the Dreamcast after March 31 on the 31st of January 2001. This decision followed a consolidated net loss of ¥51.7 billion reported in March 2001 and declining sales after early 2000.
In 2005 Sega purchased Creative Assembly known for Total War and Sports Interactive creators of Football Manager. Index Corporation was acquired in 2013 becoming Atlus while Two Point Studios joined the fold in May 2019 bringing Two Point Hospital to their portfolio.