Your browser introduces itself on every page. This is what is on that calling card.
With every request your browser sends a user agent: one line of text with the name and version of your browser and of your operating system. Websites use it to decide which version of a page to send, and analytics packages count their visitors with it.
That line is a mess left over from the nineties. Almost every user agent starts with Mozilla/5.0, including the ones from Chrome, Safari and Edge, because browsers once had to pose as each other to avoid getting the stripped-down version of a site. Chrome is slowly cleaning that up with client hints: separate fields for brand, version and platform, which you also see above.
A page picks up plenty more without asking. Your screen size, how many colors that screen handles, your preferred languages, whether you use a dark theme, how many processor cores you have and which graphics card does the drawing. On its own each detail says little. Together they make you recognizable, and that is what fingerprinting leans on.
The line of text your browser introduces itself with to every website you visit. It says which browser you use, which version and on which system.
Pure history. In the nineties websites sent their best version only to Netscape, so other browsers started calling themselves Mozilla too. Nobody dares to take it out anymore.
Yes, with an extension or through your browser's developer tools. Websites are easy to fool that way, but you tend to stand out more, not less.
Because your screen has more physical pixels than it reports in CSS pixels. A ratio of 2 or 3 is normal on phones and on laptops with a high-resolution display.
Through WebGL, the technique for 3D graphics in the browser. It passes on the name of your graphics chip so a site knows what it can draw.