Fancy text is ordinary text represented with unusual Unicode characters. It may look like a new font, but a typical fancy text generator does not install a typeface or turn words into an image. It replaces characters such as A, b and 7 with related characters that already have their own Unicode code points.
Characters, not a font file
A font is software that draws glyphs for characters. Unicode is the standard that gives characters stable identifiers. When you copy 𝐀 instead of A, you are copying a different character. The font on the receiving device decides how that character appears.
This distinction explains both the convenience and the limits of fancy text. The result can be pasted into many plain-text fields because it is text. However, its exact shape may differ, and a device without a suitable glyph may show a box.
Where the styled alphabets come from
Many popular bold, italic, sans-serif, script, Fraktur and double-struck styles come from Unicode’s mathematical alphanumeric symbols. Those alphabets were encoded to distinguish mathematical meaning, not to provide social-media fonts.
Other effects use different parts of Unicode. Circled letters come from enclosed alphanumerics. Fullwidth forms were created for East Asian typography. Underlines and strikes can be produced with combining marks that attach to a preceding character.
What fancy text is good for
Decorative Unicode is most useful for short, non-essential text: a display name, brief heading, label or visual accent. It can help a phrase stand out in a field that does not provide rich-text formatting.
It is a poor replacement for semantic formatting in a website, document or email editor. If a real heading, bold button or emphasis control exists, use it. Semantic formatting is easier to search, translate, edit and understand with assistive technology.
Why some styles are incomplete
Unicode was not designed as a collection of decorative alphabets. Superscript and subscript sets, for example, exist for particular linguistic and technical needs and do not cover every Latin letter. A careful converter leaves unsupported characters unchanged instead of substituting unrelated symbols.
A practical test
Before publishing fancy text, paste it into the exact field where it will be used. Check it on a second device if the text matters. Keep a normal-text version nearby whenever readability, accessibility or search is important.