Bubble text uses the Unicode "Enclosed Alphanumerics" block, which contains letters and digits pre-drawn inside a circle as a single character, Ⓐ, Ⓑ, Ⓒ and so on. Because each bubbled letter is one code point rather than a letter plus a separate circle graphic, the whole word stays copy-pasteable as plain text anywhere Unicode is supported. Coverage is uneven by design: the block only goes up to the digit 9 (no bubbled 0 for the 1–9 style, though 0 has its own circled zero) and has no bubbled punctuation, so symbols and higher numbers pass through unchanged.
The Unicode block used for circled letters only defines glyphs for A–Z, a–z, and 0–9, punctuation, emoji, and accented letters have no circled equivalent, so they're left as-is.
Unicode does include a separate 'negative circled' block with white-on-black filled bubbles for uppercase letters and digits, but it has no lowercase coverage, which is why this tool sticks to the more complete outlined version.
Yes, since it's standard Unicode text, just check the app's character limit, as bubbled text takes the same character count as plain text but can look visually wider.