The mere existence of a competitor doesn't matter in competition law.

Other oil companies existed, but the government still broke up Standard Oil.

Other browsers existed, but the government still made Microsoft make certain API's available to other browser makers.

This is a false equivalence. Standard Oil was at the top. Apple is clearly not. In the USA, they may very well be, but I doubt the margin is very high. https://images.app.goo.gl/hLr1zAGs7Wq5TG46A A lawyer will say that Apple though in the lead as a brand, is still second to Android in terms of market share, no matter how fragmented the market is.

This might be the same in other western countries as well. But overall Android would still have more devices running it than iOS.

Again, competition law does not focus on the existence of competitors (true monopolies exist only in high school microeconomics).

If a company has durable (even if not a literal monopoly) market power due to its anticompetitive behavior and exclusionary conduct, that is grounds for taking antitrust action.

It's hard to switch to a different smartphone, due to anticompetitive actions taken by Apple such as their app store monopoly and making it harder to distribute web apps - switching to Android means losing all the apps you've purchased.

It is a lot easier to switch to a different brand of kerosene fuel, so antitrust action against Apple is justified despite them having a lower market share than Standard Oil did.

I'd suggest reading a textbook on US antitrust law and the court decisions that have led us to where we are, because this is a large topic that goes well beyond market share.

It's doubtful the focus would be there, it'll be on how locked down their ecosystem is, the app store, API's, proprietary ports etc.