It's interesting to examine this website with the thousands of trademarks for "X". Some are extremely stylized while others are just the letter X. Some are specific to particular colors. Some cover very narrow areas. Microsoft has a lot of trademarks here, but Apple shows up too.
Elon Musk also did own x.com in the 1990s. [1] It was sold, but he repurchased it several years ago and left the page to resolve to a simple "x" index. [2] This usage was before Microsoft's 2003 registration.
https://trademarks.justia.com/search?q=X