Pretty much any mobile industry stat. Most have Android at a bit over 50% in the US and Apple at a bit under. Worldwide, Apple is under 20% market share. Remember, iOS is just Apple whereas Android is Samsung + LG + HTC + Google + Oppo + OnePlus + etc...
Examples:
https://www.counterpointresearch.com/us-market-smartphone-sh...
https://www.statista.com/statistics/266572/market-share-held...