Partially. The Blink browser engine used in Chromium is a fork from WebKit but it’s diverged quite a bit in some ways I believe. But there’s a lot more that goes into the project. For example, V8, the browser’s JavaScript engine.

The thing is, fonts are copyrightable but typefaces aren’t. Typefaces are the symbols, fonts are the files that contain all the symbols along with the formatting and everything else that let you use the typefaces in software. So he probably can’t copyright the symbol itself and it’s doubtful he could get a trademark on it either. But at the same time, copyright is also weird in that if he made an image and had that X in it, he would have the copyright to that specific image. But that’s only insomuch as anyone else would also own the copyright of an image they made with the stupid X in it.

It’s literally just a Unicode character.

From elsewhere:

𝕏 is a generic Unicode character known as “mathematical double-struck capital X.”

The API for Reddit was free. I think for sites like Imgur, Redgifs, etc they paid some pretty reasonable fees. It’s worth noting that many of them had revenue sharing agreements and trademark licenses with Reddit that were terminated after spez became CEO.