How upside-down text actually works
There's no font that physically rotates text - instead, each letter is swapped for a different Unicode character that happens to look like the original flipped 180 degrees (a lowercase "e" becomes the schwa character "ǝ", for instance), and the whole string is reversed character-by-character so that it reads correctly once visually mirrored. Because this relies entirely on finding real look-alike characters that already exist in the Unicode standard, a handful of letters and symbols without a good flipped counterpart are left unchanged rather than swapped for something that wouldn't actually look right upside down.
Why some letters flip more convincingly than others
Letters like "p"/"d" and "b"/"q" flip into each other almost perfectly, since their shapes are already natural mirror images. Others, like "s" and "z," have no distinct flipped Unicode counterpart and are simply reused as-is - visually they happen to look similar enough right-side-up and upside-down that this isn't very noticeable in practice.
Where this text works
Because the output is just standard Unicode text under the hood, it pastes correctly anywhere plain text is accepted - social media bios, usernames, captions, chat apps, and most documents. A small number of very old systems or restrictive apps may render an uncommon character as a blank box, but this is rare on modern devices and platforms.
Common uses
A novelty effect for a social media bio or post, a playful signature, or simply a fun way to flip a short message or name for a joke or attention-grabbing caption.