There is no real "upside down" character — this is a lookalike trick#
Unicode has no dedicated block of upside-down letters. What this tool actually does is substitute each letter with a different, existing character that happens to look like the original rotated 180 degrees — lowercase "e" becomes "ǝ" (a real phonetic character called schwa), lowercase "y" becomes "ʎ" (from the International Phonetic Alphabet). A handful of letters, like "s", "o", "x" and "z", already look the same upside down, so they stay unchanged. This is the identical approach every "flip text" generator uses; there is no more "correct" alternative.
Why the whole word reverses, not just each letter#
Flipping a physical piece of paper with text on it upside down does two things at once: it rotates every letter, and it reverses their left-to-right order, since what was on the right ends up on the left after the flip. This tool reverses the character order for the same reason — flipping only the letters while keeping their original order would not actually look like an upside-down flip of the whole phrase.
Where this shows up in practice#
The most common uses are playful: a social media bio, a caption, a chat message, or a name meant to stand out. Because it relies on font coverage for a set of less common Unicode characters, some of them (particularly the capital-letter approximations, which are rougher matches than the lowercase ones) may render as a missing-glyph box on a device or app whose installed fonts do not include them — the same limitation every Unicode-character-substitution trick has.