By far the biggest tracking offender is Javascript. Enabling it could reveal your operating system, cpu/gpu architecture, screen resolution, draw a precise and unique canvas fingerprint, etc. There are also mutable browser headers like user-agent and of course your IP address. However, the more advanced and insidious tracking is based on your behavior - what time you're active, what wifi networks are in range, who you communicate with, what is your writing style, and so on. Most of that collection happens on mobile phones, so I strongly advise against signing in on Android/iOS devices if you don't want to be tracked across the Web and beyond, or using telemetry-free open source mobile operating systems altogether.