X/Twitter has no custom font picker, so stylized bios and posts are built from ordinary Unicode letters pulled from other character blocks, bold and italic mathematical alphabets, Gothic/Fraktur, circled letters, and more, that still behave as plain text once pasted in. This tool renders every available style from a single line of input so you can pick your favorite without regenerating each one separately. One quirk worth knowing: several of these styles live outside the Basic Multilingual Plane and are represented internally as surrogate pairs, which can make them count for more of X's character limit than the same text in plain letters.
Styles like Bold, Script, and Fraktur come from Unicode's Mathematical Alphanumeric Symbols block, which sits outside the Basic Multilingual Plane and is stored as a surrogate pair, depending on how a platform counts characters, that can mean each styled letter counts as two instead of one.
Yes, since it's all plain Unicode text, it pastes the same way into your display name, bio, and individual posts.
Each style only maps the specific letters and digits it has an alternate form for, anything outside that set, like punctuation or emoji, is passed through exactly as you typed it.