Short answers, pulled from the story.
MS-DOS was derived from 86-DOS, written by Tim Paterson of Seattle Computer Products in roughly six weeks. Microsoft acquired 86-DOS 1.10 in July 1981 and hired Paterson in May of the same year, then licensed the renamed software to IBM as PC DOS 1.0.
MS-DOS 6.22, released in 1994, was the last version sold as an independent product. Development of MS-DOS as a standalone operating system formally ended in 2000, with versions 7 and 8 functioning only as background components inside Windows 9x.
Microsoft was accused of leaking details about future MS-DOS versions to create industry uncertainty around DR-DOS, a practice known as FUD. Microsoft also embedded the AARD code in Windows 3.1, an obfuscated routine that detected whether Windows was running on genuine MS-DOS and displayed an error message in beta versions when it was not.
Stac Electronics sued Microsoft for patent infringement after MS-DOS 6.0 shipped with a disk compression utility called DoubleSpace that used a compression algorithm from Stac's Stacker product. Stac won, and Microsoft released MS-DOS 6.21 with disk compression removed, followed by version 6.22 with a different algorithm called DriveSpace.
Microsoft released source code for MS-DOS 1.25 and 2.0 on GitHub under the MIT License in 2018. Source code for MS-DOS 4.00 followed in April 2024 under the same license. Program manager Rich Turner stated that other versions could not be released due to third-party licensing restrictions.
Windows 11 removed the NTVDM component that had allowed earlier 32-bit Windows versions to run DOS applications in emulation. This ended any direct association between MS-DOS and active Windows releases after 36 years, as Windows 11 is offered exclusively in 64-bit versions.