So, for one, "monopoly" doesn't technically concern "literally only one", but that it has a "dominant market share". Monopoly is a term that feels a bit too narrow, but it's understood to apply to the issue a bit more broadly in law and legal discussions. In many cases, you're understood to be a monopoly if you have say, 70% of a market or more.
Google is over 85% of all search traffic. Now, you might argue as Google has, that sure, anyone can change their default search to Bing. However, the issue with dominant companies is the network and second order effects of how they impact everything else.
For example, Google's search data is primarily refined by "the fact that everyone is searching with Google". So the more popular Google becomes, the better it's data becomes... and the less possible it is for other search engines to compete. It's not a talent problem, it's a data problem.
But another big aspect is the other side of a search transaction: The websites you find with Google. Since Google is 85% of search, Google search rankings determine if businesses survive, pretty much singlehandedly. It doesn't matter if you're first on Bing, because people who find you on Bing aren't enough to sustain your business. You must be findable on Google.
Often, that means businesses must do business with Google: The first "search result" on Google search is almost always a paid advertisement. Businesses are forced to do business with Google to exist, and the fact that other search engines exist is... mostly irrelevant to them. This also means Google can dictate what websites can and can't display, what technologies they must and must not use, etc. AMP is terrible but Google was giving preferential treatment to websites with AMP, so AMP is understandably all over the place now.
Mobile operating systems is an intriguing one, because believe it or not, if you understand the market they're in Android is a total monopoly. Android is 100% of the mobile operating system market. The default question to ask is "what about iOS", and the answer is simple: iOS isn't on the market. Because the market isn't consumers, it's phone manufacturers, and Apple iOS is only available for the Apple iPhone.
If you're Samsung or HTC or Lenovo or Huawei or ZTE or Motorola, you have one option to sell phones: Sell Androids. Sure, Huawei has forked Android because it got banned by the US, but it's still Android. The only competitor in the mobile OS space that had any traction at all was Windows Mobile and it's dead. If you go into a cell carrier store today, they'll sell you an iPhone, or they'll sell you two dozen phones that all run the only operating system on the market: Android.
Android has an "other side" aspect too: App developers. Even if iOS exists, businesses have to develop Android apps to reach consumers on mobile devices, and that means they have to do business with Google. And not just app development companies either. Imagine if Allstate Insurance said their app was only available on iOS: Pretty much every business in every category of industry ends up having to do business with Google. And that's a monopoly.
Google isn't a monopoly in cloud providers, it's actually in like fourth place. Apparently they're just... not good at everything. *shrug*