Fonts or characters?
None of these tools changes the font on the page you paste into. They replace each letter with a different Unicode character that happens to look bold, italic, circled or flipped. That is why the result works in Instagram bios, X posts, Discord names and WhatsApp messages where formatting buttons do not exist. The trade-off is that screen readers may spell the characters out one by one and search engines treat them as symbols, so use styled text for emphasis, names and headings rather than whole paragraphs.