Amazon is the largest online retailer in the world. It's also the largest cloud infrastructure provider, second-largest smartphone manufacturer, and largest music streaming service on earth.
While Amazon doesn't have a monopoly on any of these things, it has enough market power to be considered a monopolist by some antitrust authorities.
Google is the most popular search engine in the world. So, yes, it has a monopoly on search. However, there are no other significant players in this space that have a significant market share. Google has 90% of all searches conducted in the United States and 80% globally—meaning that if you do a Google search for "best restaurants," there's a 90% chance you're going to end up at one of its properties: Search results pages with ads from Yelp or TripAdvisor are only going to show up as part of your results if those companies pay Google for each click; otherwise, they'll just appear as part of your regular search results (with no paid ads).
So why does this matter? It's true that Google isn't actively trying to prevent other people from using their products or competing with them directly—they're open source! But what makes them different from other kinds of monopolies is that they have complete control over how users interact with their product through data collection and manipulation; no one else can offer anything similar because they don't have access to the same amount (or type) of data about users' behavior patterns.
For example, imagine if every time someone searched for "dentist in New York City" Facebook showed them an ad saying "Click here!" next time you visit NYC."
This would be annoying but not very harmful because Facebook doesn't collect enough information about us over time so our behavior patterns wouldn't change much after seeing these ads regularly enough -- but since our browser history is linked directly back into our email accounts on Gmail/Google Apps etc., these companies can use algorithms based off both metadata from past searches plus behavioral data collected via cookies which means we'll always see relevant ads regardless whether we're looking specifically for something related to dental care or not (more like when Netflix recommends movies based off shows watched).