TikTok does not let you choose a font for your bio or name. These styles are special Unicode characters that look like a different font, so you can paste them anywhere text is allowed.
Yes. They are normal Unicode characters, so TikTok accepts them in your name, bio, captions and comments.
Some old phones do not have every Unicode character. If a style shows boxes, choose another one: bold, italic and script work almost everywhere.
Yes. You can paste them on Instagram, WhatsApp, Facebook, YouTube, X and most apps.
No. Unicode only has fancy versions of Latin letters and numbers, so other alphabets stay the same.