It's not exactly a secret that Google keeps a large amount of personal data on all of its users. The subject has even managed to generate controversy in the past, including some pointed questions from US Senator Al Franken about Google's practices with the data of students who used Chromebooks with Google's suite of educational and classroom administration software. Thankfully, Google's security practices are the gold standard for the industry, making hacks quite unlikely and leaving your personal data mostly safe. Still, given the strange things Google learns about you, your life, and the people around you, it can be quite interesting to take a trip through the search giant's archives of your life, and there are a few portals through which you can do so.
For starters, try firing up the Google Photos app on your phone, or going to the website. You'll notice that Google automatically figures out who you're with in photos, if it can, and also logs where and when each photo was taken, as long as that information was available on the device that shot the photo. This makes reliving days gone by a little more intensive. On the subject of days gone by, there is also an option to have your phone record your location history and make it viewable on Maps. The data is, of course, quite private, but can give you some insight as to your past travels. Usage stats for that same device, down to time spent within apps and when that time was spent, are also available in a section of the account dashboard, which you can visit through the source link.
You can also check out all the Android apps that you've ever installed, even as far back as your very first device. For those that use Google+ frequently, the number of +1's you've handed out over the years is available. The full history of your commands to Google Now and Google Assistant is available through the Audio tab on that page, and you can even play back the audio recordings. That extensive audio history also includes any time you've used the dictation features built into Google Keyboard, now known as GBoard. All of this data and then some is available via that dashboard. The data can be managed, and you can even opt out of data collection on an itemized basis.