How text effects work
There are 3 techniques behind these effects. Character swaps replace each letter with a look-alike, such as ɐ for a in upside down text. Combining marks add a line through or under each letter, such as U+0336 for strikethrough. Reordering reverses the text for backwards and mirror writing.
Where text effects work
Text effects work in any app that accepts Unicode: Instagram, X, Discord, WhatsApp, TikTok and most games. Combining-mark effects such as underline can shift slightly between fonts.