The cute fonts you see in bios and captions are not fonts. Nothing on your phone changed typeface. They are ordinary Unicode characters that happen to look like styled letters, and knowing that explains everything about where they work and where they fail.
Here is what is actually happening and how to use it without breaking anything.
They are characters, not formatting
A social bio in flowing script is not italic text. It is a sequence of characters from a mathematical alphanumeric block, or a similar range, that were defined for equations and happen to resemble a cursive alphabet.
The same is true of the bubble letters, the outlined letters, the small capitals and the upside-down ones. Each is a separate set of characters that a font on your device knows how to draw.
That is why they survive being copied and pasted between apps, and it is also the source of every problem they cause.
What breaks, and why
Screen readers. This is the significant one. A screen reader announces characters, and a name written in mathematical script is read out as a string of descriptions rather than as a word. A bio in decorative characters can be effectively unreadable to someone using accessibility tools.
Search. Text written in these characters does not match a search for the ordinary spelling. A username in script letters is not findable by typing the name.
Older devices and some apps. A device without the right font draws a blank box. Less common than it was, still real.
Some platforms strip them. Certain fields reject characters outside a basic range, so the text either does not save or arrives as gaps.
Use them for decoration, not for your name
The distinction that avoids almost every problem: put decorative characters where they are ornament and plain text where they are information. A bio can have a line of script under a plain-text name and lose nothing. A display name written entirely in script letters costs you search, accessibility and the ability of anyone to type your name to find you. If someone cannot find your account by searching for your own name, the styling has cost more than it added. Keep the name readable and decorate around it.
Where they actually work well
A single line in a bio, under plain-text essentials. This is the intended use and it looks good.
Captions, where the text is read once and not searched.
Dividers and frames, made from symbols rather than letters. These carry no meaning to lose, so nothing is harmed by them.
Sparingly, for emphasis, in the way you would use italics if the platform offered them. One phrase, not a paragraph.
Getting them onto an iPhone
There is no built-in font styler on iOS, because as established there is no font involved.
The two practical routes are a keyboard app that offers styled text, or Text Replacement for phrases you use repeatedly. Text Replacement is in Settings, General, Keyboard, Text Replacement: add the styled text as the phrase and a short trigger as the shortcut, and it expands anywhere on the phone and syncs across your devices.
Text Replacement is the more reliable of the two, because it works in every app including the ones where custom keyboards are blocked, such as password fields.
Building a small set you actually use
The way people usually acquire these is by copying from a web page each time they need one, which is slow and is why most bios end up with whatever was at the top of the list.
A better approach takes ten minutes once. Pick three or four styles you genuinely like — one script, one bold, one small-caps, and a divider made of symbols is a reasonable set. Save each as a Text Replacement with a short trigger you will remember.
The triggers matter more than they look. Use something you would never type by accident, since Text Replacement fires anywhere on the phone. A leading punctuation mark works well, because ordinary writing rarely begins a word with one.
After that you have your own styles available in every app, on every device signed into your Apple Account, without opening anything. It also stops the slow drift where a profile ends up in four different styles because each was copied from a different page on a different day.
A note on mixing scripts
Combining styled letters from several different blocks in one line is where things go wrong visually. Each block was designed independently, so letter widths, weights and baselines do not match, and a line built from three sets looks uneven in a way that is hard to point at.
Pick one style per line. It is the difference between a bio that looks designed and one that looks assembled.
Quick answers
Will these work on Android and desktop?
Mostly, since they are standard characters. The exact appearance varies with the font available, and a few are drawn quite differently across platforms.
Can I use them in a filename or an email address?
Filenames usually yes, email addresses no. Address fields are restricted to a basic character set.
Do they affect my reach on social platforms?
They can, indirectly, since text that cannot be searched or read by accessibility tools is text the platform understands less well. Keep the important words plain.
Why does my bio show boxes on my friend's phone?
Their device does not have a font covering those characters. Choose a more common style, or move the styling to a place where losing it does not matter.
Where to get it
Kawaii Keyboard & Wallpapers includes styled text and decorative symbols alongside the keyboard itself, which saves collecting them by hand from a web page.
Keep your name plain and put the styling on the line underneath. That single rule solves most of it.