The Define step takes all of the findings from the Discovery stage and answers the 2 key questions:
We will normally recommend a phased approach to implementation, with an initial MVP and then after that has been tested your customers we can add further features.
Taking this approach keeps the costs down and get a version out to your customers as quickly as possible.
Unless there is a particular reason, such as you will be supplying both the hardware and software, we always recommend developing for both Android and Apple at the same time. In the UK and most developed countries, both Apple and Android have approx. 50% of the market.
It does not normally add too much to the overall time to have both platforms and the benefits in reaching your customers should not be underestimated.