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As ISO explains
on their page:
1) ISO 9001 is for design and/or production of product as well as design and/or production of services,
2) ISO 20000 is for service management, and
3) ISO 27000 is for IT security methods
Particularly since the 2015 version of ISO 9001 focuses on identifying and controlling risks (something I understand IT people know about) if you are making apps I would think 9001:2015 is most appropriate. How might it improve your strategy? I suggest through reduction of errors, shorter time-to-market, and shorter time in beta via your recognized risks and controls put in place to minimize their impact on your operations and product.