Google My Activity is a great place to start if you want to find out what Google knows about you. Here, you can see all the searches you've done, the websites you've visited thanks to Google Analytics, and even more if you're signed in to a Google Account. Google uses machine learning algorithms to classify information and provide you with great suggestions. It could be a suggestion from YouTube, or a product that you might be interested in buying.
Google also keeps track of your YouTube searches. Every time you open the Google Maps app on your Android mobile phone, Google knows your location and it's usually very accurate. Even if you don't use any Google product directly, they're still trying to track everything they can about you.
Every web search you have performed enters a database and is analyzed so that Google can improve your experience. It may surprise how much it knows. For example, Google uses the BSSID information from your WLAN access point to get an approximation of your location, even when GPS and WiFi are turned off.
Google even allows users to download all their data, including photos, emails, contacts, bookmarks and more. This way, you can copy, backup, or even move them to another service. It's interesting to see the variety of things that Google thinks you're interested in, especially in a personal account.
In addition, you can set your default location with the Google Maps app. By allowing Google to collect all of this information, hundreds of thousands of advertisers can bid to show you ads based on your sensitive personal data.
So essentially, when you're using WiFi and GPS, Google's BSSID database is updated with a geographic location associated with that BSSID. That all changes if you're signed in to a Google Account - the most convenient way to access personalized information on all your devices.
If you want to find out what Google knows about you and how it uses this data, it's important to understand how it collects and stores this information. Knowing this will help you make informed decisions about how much data you want to share with them.