Monopolies have nothing much to do with government, they're about being the only provider in a market. If Apple controls less than half, they're not the only provider even within the US, and that only makes it weird for you to say - effectively - only the US matters, other countries don't.

Google controls Android which Samsung use, and the terms on which they're allowed to use it if they still want to allow Google apps. That means it's not Samsung vs Apple, it's Google vs Apple, especially when my reply was to someone comparing Google to Apple, not Samsung to Apple.

It is not Google vs Apple. No court would take that case. Google hardly has any market share of phones (Pixel) and Apple doesn't sell OSes

Apple and Samsung are in direct competition. Apple has >50% of the smartphone market in the USA. Samsung has less then 25%, every one else has even less. Google has < 1%. That Samsung happens to use Android is irrelevant. FWIW Samsung has its own app store and plenty of distinguishing features from every other phone.

The rest of the world has its own markets. If the UK wants to sue Apple or Google for being a monopoly they only care about the UK market, not how it's selling in Indonesia. In other words it doesn't matter one wit if some company has a large market share in the world, monopolies are only enforced in a specific country for that country's market, not the world market. You quote iOS has only have 20%, but as that is a world wide number it's entirely irrelevant and pointless. If iOS had 100% market share in Singapore but only 5% marketshare in the world it would be Singapore suing because of the Singapore market. There is no one who can sue on behalf of the world.

I don't understand why you're trying to pick Samsung out. If Google strongarm Samsung and Motorola and LG and say "you can't put Google apps, Google Maps, Google play store on your phones anymore and can't get Android updates from Google unless you XYZ" that has a greater reach than Samsung alone has, doesn't it? And that is a reach that Google can have, and Samsung, Motorola, LG alone cannot have, right?

And by reaching over multiple Android phone sellers, that has at least a similar, but likely much greater reach than anything Apple can do, doesn't it?

Whether a company can be sued for being a monopoly in a given jurisdiction isn't so interesting to me, as whether they can influence ~80% of the worldwide smartphone market; If they can do that but you can't sue them for being a monopoly in Tuvalu that has no bearing on anything interesting.