Luuk,
a very good question! :)
Most Noto fonts are designed by a team at Monotype, under the direction of Steve Matteson who designed the Latin characters in Noto, but who also made earlier font projects for Google, including Droid Sans and Open Sans.
The Noto Sans CJK fonts are an exception: they are actually made by Adobe as part of the Source Han Sans project. Source Han Sans (
https://github.com/adobe-fonts/source-han-sans ) and Noto Sans CJK are the same fonts released under different names. The Latin characters in Source Han Sans were designed by Adobe designer Paul Hunt as part of his Source Sans Pro, also opensource (
https://github.com/adobe-fonts/source-sans-pro ). The Noto Sans CJK fonts include Japanese-style Latin characters (fullwidth and halfwidth), and those were only available in the Adobe design.
Also note that the Noto Sans CJK fonts are available only as OTF, while most Noto fonts are primarily available as TTF. This has also to do with the fact that it's Adobe who produce them. Noto Sans CJK aka Source Han Sans are actually a very advanced set of fonts on the technical level — something like that has never been done, (a set of stylistically unified CJK fonts for SC, TC, K and J), with a full character set coverage. Let alone in the opensource realm.
The Noto font project is a collaboration of Google, Monotype and Adobe.
Best,
Adam Twardoch