They're characters, not fonts
TikTok doesn't let you change the font of your bio or name. These styles work anyway because they use special Unicode characters that look like bold, cursive or outlined letters. TikTok shows them like any other text, so they paste into your bio, display name, comments and captions.
Where to use them
Fancy text works best in small doses: your display name, one line of your bio, or the first words of a caption. Your username can only contain letters, numbers, underscores and periods, so fancy characters won't work there.
Things to know
Screen readers may read fancy characters letter by letter or skip them, and TikTok search can't match them, so keep important words in normal text. Some styles use two characters per letter, which counts more toward the 80-character bio limit. Check your text with the character counter before you save.