Every character you have ever typed — every letter, digit, emoji and accent — is stored by your computer as a number. Unicode is the worldwide agreement about which number means which character. Before Unicode, different computers used different, incompatible numbering systems, and text that looked fine on one machine turned into garbage on another.
One standard for every language
Unicode assigns a unique number, called a code point, to more than 150,000 characters covering virtually every writing system on Earth: Latin, Cyrillic, Arabic, Devanagari, Han characters, historic scripts, mathematical symbols and emoji. Code points are written like U+0041, which is the capital letter A.
Where the fancy letters live
Along the way, Unicode also encoded thousands of styled letters for specialist use. Mathematicians needed bold, italic, script and double-struck alphabets as distinct symbols — in a formula, 𝐀 and 𝒜 mean different things — so the standard includes entire parallel alphabets in the Mathematical Alphanumeric Symbols block. Phoneticians got small capital letters. Typographers got circled and fullwidth forms.
Text style generators repurpose those blocks: when you convert "Hello" to "𝗛𝗲𝗹𝗹𝗼", each letter is swapped for its bold mathematical cousin. The result is ordinary text — just built from different code points — which is why it survives copy-paste into places that strip formatting.
Why it works everywhere
Because Unicode is universal, a code point means the same thing on every modern phone, browser and operating system. As long as the device's fonts include a glyph for that code point, the styled character renders correctly. That single fact is what makes copy-paste fancy text possible at all.