That's difficult, without openness or protectionism. How do you compete with iOS without supporting iOS apps, iCloud, and iMessage blue bubbles? The switching costs (practical, and psychological) are too high.
You build good alternatives for each of those.
Ask Microsoft, who tried developing Windows Mobile after the market was taken over by iOS and Android. Or who still tries competing with Chrome without much success, despite it being pushed on Windows as the default, with annoying settings and ads included. Nevermind that they gave up developing their own engine because they couldn't keep up.
Nokia and BlackBerry found that out as well.
Android is at nearly 70% OS market share: https://gs.statcounter.com/os-market-share/mobile/europe/
[I say this as a keen iOS user]
The statement doesn't make sense because people don't keep 2 phones in their pocket, or 2 smart watches, and rarely have 2 tablets that are actively used. And people definitely move between ecosystems. Choosing one excludes the other, and they have equivalents for each product type.
Hence they are direct competitors.
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The real problem is that the mobile OS & app store market is a stable duopoly.