Where these bubbles come from
Outline bubble letters (Ⓐ ⓑ ⓒ) are “circled Latin letters”, added to Unicode in 1993 in the Enclosed Alphanumerics block, originally for numbered lists in East Asian typesetting. The solid black versions (🅐) and the squared ones (🄰) are newer, from the Enclosed Alphanumeric Supplement, so they are drawn by emoji fonts on most phones.
Solid bubbles and squares only exist as capitals, so this generator shows every letter as uppercase in those styles.
Want real bubble letters for a poster?
For signs and graffiti-style drawings, bubble lettering is usually drawn by hand around a thick skeleton line. The copy-paste versions here are for screens.