As an example, just take a fairly normal life situation like going out for dinner with some friends, and think about how much of it goes "through" Google:
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One of your friends sends an email to 6 others, to ask if everyone's free for dinner on Friday. 5 of the 7 people involved use gmail or a google apps address.
You've never heard of the restaurant they're suggesting, so you search for it on Google to see what kind of food it is.
You click to the restaurant's site. It uses Google Analytics, so even though you're no longer on Google, it still knows the exact path you take through the site while you're "outside".
You decide the restaurant looks good, and enter the dinner into Google Calendar.
On Friday, you use Google Maps to get to the restaurant, so Google knows exactly where you were before, what time you left, and the route you took. While you're driving, maybe you send a couple of text messages using the Google voice assistant.
At the restaurant, it turns out your friend Doug is there, even though he wasn't part of the emails. During dinner, you're all trying to remember the name of that movie where Shaq plays a genie, so both you and Doug grab your phones and google for phrases like "shaq movie genie" at about the same time. Even though Doug wasn't included in any of the planning, Google now knows that you're almost certainly together, and what you're talking about.
You finish your meal and pay via the restaurant's Square system, which emails the receipt to your gmail address. Google now knows exactly what you ate, and how much you paid for it.
You use Google Maps again when leaving, telling Google exactly how long you stayed at the restaurant and where you're going next.
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I didn't even push that very far. There are multiple other things I could have easily added, and you can do this with almost any situation. It's quite insane how much Google knows about what people are doing all the time, and the level of detail they can get by combining these things.