The developer preview build, codenamed Android “M”, was unveiled and released at Google I/O on May 28, 2015, for the Nexus 5 and Nexus 6 phones, Nexus 9 tablet, and Nexus Player set-top box, under the build number MPZ44Q.[5][7] The third developer preview under build number MPA44G was released on August 17, 2015,[8] later updated to MPA44I, and brought fixes related to Android for Work profiles.[9]”Marshmallow” was officially announced as the release’s name the same day.[3]

On September 29, 2015, Google unveiled launch devices for Marshmallow: the LG Electronics-produced Nexus 5X, the Huawei-produced Nexus 6P, and the in-house Pixel C tablet.[10][11] Android 6.0 updates and factory images for Nexus 5, 6, 7 (2013), 9, and Player were released on October 5, 2015,[12] with over-the-air updates following shortly after. Older Nexus devices, such as the Nexus 4, Nexus 7 (2012) and Nexus 10 did not receive an official update. On October 14, 2015, LG announced that it planned to release Marshmallow for its flagship LG G4 in Poland the following week, marking the first third-party device to receive an update to Marshmallow.[13] On December 7, 2015, Google released the Android 6.0.1 update and factory images for Nexus 6P, Nexus 5X, Nexus 6, Nexus Player, Nexus 9, Nexus 5 and Nexus 7 (2013) which added support for more emoji as well as security updates.[14]

Source: https://en.wikipedia.org