Short answers, pulled from the story.
The Mass in B minor, BWV 232, was completed in 1749, the year before Bach's death. Scholars including Christoph Wolff place the assembly of the final sections in 1748-49, with Yoshitake Kobayashi dating the Symbolum Nicenum to August through October of 1748 based on Bach's handwriting.
Bach began the Mass in 1733 to present to Augustus III of Saxony, hoping to obtain the title of Electoral Saxon Court Composer at the Dresden court. He expanded it into a complete Latin Ordinary in the last years of his life, possibly with an eye toward preserving his finest vocal music in a durable format.
The Mass in its entirety was never performed during Bach's lifetime. The first fully documented complete public performance took place in 1859 in Leipzig, led by Karl Riedel and the Riedel-Verein, over a century after Bach's death.
As of 2022, a database lists over 350 complete recordings of the Mass in B minor, featuring many different performance styles and forces. The first complete recording was made in 1929, with a large choir and the London Symphony Orchestra conducted by Albert Coates.
In 2015, Bach's personal handwritten manuscript of the Mass in B minor, held by the Berlin State Library, was added to UNESCO's Memory of the World International Register, a project dedicated to protecting and preserving culturally significant documents.
The Mass in B minor consists of 27 movements. Despite the work's name, only five of those movements are actually in B minor; twelve movements, including the final movement of each of the four major sections, are in D major.