How flipped text works
Plain text has no rotate setting, so the generator swaps every letter for a character that looks turned upside down, then reverses the order so the last letter comes first. Read with the screen held normally, ʇxǝʇ looks like nonsense; turn the screen around and it says text.
Many turned letters come from the International Phonetic Alphabet, which uses ɐ, ɥ, ɯ, ɹ and ʌ for speech sounds, along with ʎ, the turned y. Others are borrowed from elsewhere: ∀ is the "for all" sign from logic, ⅁ and ⅄ are turned sans-serif capitals from the Letterlike Symbols block, and capitals like ꓭ and ꓤ are letters of the Lisu script.
What flips well and what does not
Letters like o, s, x and z look the same either way, and pairs like b and q or d and p simply swap. Punctuation turns too: ? becomes the Spanish ¿, ! becomes ¡, and a period moves up to ˙.
Numbers are the weakest part. 6 and 9 trade places, but for 2, 4 and 7 the generator uses look-alikes: ᄅ from Korean Hangul and ㄣ and ㄥ from Bopomofo, a Chinese phonetic alphabet. Emoji and accented letters have no flipped twins here, so they stay as they are and only move position.
Fun uses and fair warnings
Upside-down text is great for jokes, "down under" posts about ɐᴉlɐɹʇsn∀, puzzle hints, April Fools messages and names that make people tilt their heads. Need text flipped left to right instead? The mirror style here and the backwards text generator do that.
Because each letter is really a phonetic or math character, screen readers may read flipped text as a string of odd names, and search may not find the words. Keep it to a short line, and see the fancy text generator for styles that stay readable.