This gets decided by preference far more often than by evidence. The evidence is usually available and usually decisive.
Start with where your users are
Not global market share — your market's share. Android dominates by volume worldwide and overwhelmingly in India, Southeast Asia, Latin America and much of Africa. iOS leads in the US, UK, Japan and Australia, and leads by more in the segments that pay for software.
Then your revenue model
Subscription and in-app purchase revenue skews heavily to iOS. Advertising and volume-driven models often favour Android. If you are monetising directly and your market is North America or Western Europe, iOS first is usually right.
Then your distribution
Enterprise and field deployments frequently run managed Android fleets, which settles the question regardless of everything above.
When the answer is "neither, do both"
Where the split is genuinely even and the interface is not your differentiator, sequencing is the wrong frame — React Native gets you both stores on one budget, and that is the cheaper answer than launching twice.
What we'd tell you
We build iOS and Android natively and React Native across both. That means we have no reason to push you toward one, and the answer in your written scope will be argued rather than assumed.