google knows everything.  it's plugged into the data sharing metrics with DARPA.   every single piece of internet traffic worldwide, flows through DARPA's server system.  it is the top of the food chain in terms of touching all files that happen on / through / over / under / above / below / --inside-- the internet.
the internet in general is largely mis-understood. it's a distributed system of many computers but with centralized nodal points, that facilitate and regulate and manage the flow of information. at the base of that mesh of "the internet" is the DARPA servers and their systems.
thats what happens when you invent something, and build it so you always have access.
nothing you do on the internet is private, ever, really, but that's not even the point :)
if you ask me, we are inside a black hole, and created the internet as a means to communicate a bit better in a singularity event, but this would take me a while to describe here.... and is another layer to the cake, as it were. :)
but google knows whatever it needs to know, or wants to know, because it is a creation of the defense industry and DARPA is a part of the defense industry. :)
defense from whom?  well, another story entirely, but its not from other people.  😁😆🙂🙃🙃🙃

with deference to your original question, hower - Andrew already hit the nail on the head.  I just wanted to throw this extra pickles in for fun.