Short answers, pulled from the story.
Square and Enix officially merged on the 1st of April 2003, with Enix as the surviving legal entity. Former Square president Yoichi Wada became president of the combined company, and each Square share was exchanged for 0.85 Enix shares.
Square's 2001 film Final Fantasy: The Spirits Within failed at the box office, causing Enix to withhold approval while Square recorded two consecutive years of financial losses. Sony's purchase of an 18.6% stake in Square in October 2001 stabilized the company, and the merger was formally announced on the 25th of November 2002.
Square Enix sold Crystal Dynamics, Eidos-Montréal, Square Enix Montreal, and the Tomb Raider, Deus Ex, Thief, and Legacy of Kain franchises, along with rights to more than 50 other games, to Embracer Group for $300 million. The deal closed on the 26th of August 2022.
The Final Fantasy franchise has sold over 173 million units worldwide as of March 2022. Dragon Quest has sold over 85 million units, and Kingdom Hearts has shipped 36 million units as of the same date.
Yasuhiro Fukushima founded Enix on the 22nd of September 1975 as Eidansha Boshu Service Center. Fukushima was a trained architect who built Enix around a publishing model, partnering with external developers rather than building games internally.
Square Enix restructured on the 26th of March 2013 after projecting a loss of 10 billion yen, driven by weak Western sales of Tomb Raider and Hitman: Absolution despite their critical success. President Yoichi Wada resigned and was replaced by Yosuke Matsuda, who cited overly long development cycles as a key problem.