It's very simple. You can legally have a monopoly. If you're the best at doing something and so everyone goes to you, it would be totally unfair to regulate your monopoly out of existence. So you get to keep it. After all the aim of this law is to protect the consumer, and we can assume that they all went to you for a reason.
What you're not allowed to do is to use that monopoly (market dominance) in order to enter other markets. At which point it all becomes rather murky, as to what's being normally competitive and what's unfair competition.
You don't need 100% control of a market to be defined as a monopoly, it can be less than 50%, it's about your market power.
This case is also a bit weird of course, in that Google don't charge for search - and so the customers are the advertisers. Except that the ads are only going to get seen if people use search.
The Microsoft example was a lot clearer. They were considered to have a monopoly with Windows, and were using that to push their browser. Even though Netscape had a nice business charging for a browser. Not that the law intervened in any kind of timescale that would have saved Netscape.
A more traditional monopolistic abuse might be Vanderbuilt, in the late 19th Century. He built a nice railway that was used by a bunch of steel mills. Then he decided to go into the steel industry. And those mill owners were asked to sell their mills to him at a substantial discount. If they didn't, his railway would stop doing business with them, and they'd go bust.
Or say BT, who were charging about 50p a minute for daytime telephone calls in the early 1980s. Because your alternative was not to be able to make calls.