Personal identification information like emails, phone numbers, date of birth
Device info including the devices you use to access google, models, OS, unique identifiers, and mobile network info
Location info including IPs, GPS, and data from other sensors Search queries including links you interact with and other info related to interaction with websites
Usage data including language preferences and which ads you interact with
Communication data like the time and date you made certain communications, the content and subject of the communication, voicemail messages in google voice, who you’re communicating with, when, content of chats including in hangouts/chat/meet, call quality, call duration, and other technical data
Content interaction information on how you interact with content through Google services; views, downloads, shares, likes, and comments
Youtube viewing history, including watch time, and impressions, what you do while you’re watching them, likes, and subscribes
Google apps data specific to app usages; so things like google Drive that collects data on files you upload, download, share, types of files, who you share them with, how often you access them, changes you make, and comments you leave; Gmail collects data from emails including who you’re emailing, who’s emailing you, and the email subject and contents; GCal collects all the data you input as well as how often and how you use your calendar; Photos collects data on where and when the photo or video was taken, who was in it, and other details about the photo; Maps tracks search info, routes taken, and other location-based information; Play collects data on apps you download, how often you use them, how you interact with them
Data collected through cookies that collect and store information, as well as pixel tags, local storage, browser web storage, application data caches, databases, and server logs

TL;DR if they can, they track it. We also have only some idea as to how they could be using this data to create personal profiles and social graphs for users to even link you with others in your proximity by comparing time and location data, pictures, preferences, etc.