I'd be interested to know (in generalities) what they know about you...
Google knows everything that's in my emails, which is pretty much EVERYTHING to know about me. I buy something from an online retailer and the retailer sends me a confirmation email with my name, address, phone#, CC info, etc. After a few dozen such emails they know everything about me.
Download everything Google has saved about you.
https://takeout.google.com/
I finally got around to viewing the .MBOX files from my two Gmail accounts. The files are called "All mail Including Spam and Trash.mbox," but there are very, very few actual SPAM emails in the archive. There are only about 600 emails in one of the archive as well. I created this account in 2005, but there are only 7 emails for all of 2005. Only about 50 for 2006. Some months only have one email. Other months have many. I don't really know the "who, what, where, when, why or how" emails were selected/saved by Google. Maybe a random sampling of INBOX items. Or items that have been in INBOX for X number of hours/days? Perhaps the contents of the emails themselves trigger some "save/archive this email" function. Who knows? The .MBOX file also contains all the attachments contained in the emails, with a size range from 1 to 30,000k each.
It's a lot less email then I thought there would be. It would have been really interesting if it was every email ever sent/received. I'd rather have all the email without attachments then just a small percentage of emails with attachments. Oh well. It was, however, kind of cool to see the first email I received from Gmail, (username removed):
"Congratulations on creating your brand new Gmail account at
{username}@gmail.com. Please keep this email for your records.
You can login to your account at
http://gmail.google.com/
Enjoy!
The Gmail Team"
... The second account I looked at didn't contain the "Welcome to Gmail" email. Again it's an interesting stroll down memory lane via email but with so many emails missing the archive itself is less useful than it could have been.