X has one font for everybody, and no bold or italic button. These styles aren’t fonts at all. They’re other letters.
Unicode, the standard every phone and computer uses for text, has whole alphabets of bold, italic, script and outline letters, made for math. Paste them into X and they show up in that style, in a post, a reply, your display name or your bio, on every phone and computer. They paste the same way into Threads, Bluesky, Instagram and most other apps.
A few of these alphabets have holes: some letters were in Unicode long before the rest, so they live elsewhere, and the generator fills them in. Styles that have no numbers leave your numbers as they are. Source: Unicode, Mathematical Alphanumeric Symbols.