How it works
Each letter is swapped for one that looks like it turned over, ɐ for a and ʇ for t, and the order is reversed, so the whole line reads right with your phone turned round: Hello becomes ollǝH. Digits and the marks ! ? , . ' " ( ) [ ] { } < > & and _ turn over too.
Look-alikes, not true flips
Unicode has no upside-down alphabet, so the letters are borrowed from phonetic symbols and other scripts. A few capitals, such as ꓭ and ꓘ, come from the Lisu alphabet and may show as empty boxes on older phones. Accented letters and other symbols stay as they are, the right way up.
Questions
Can it be bold or italic?
No. There are no bold or italic upside-down letters, so those boxes turn off. Underline and strikethrough still work.
Can I flip it back?
Not by pasting it in again: that puts the letters back in order but leaves them turned over. Keep your original text.