{emoji 'rocket'} writes <span class="pad-emoji" role="img" aria-label="rocket"> around the emoji, so a screen reader names it instead of reading out a code point - and label= gives it another name to say. The shortcodes are the common ones of GitHub and Slack, a few hundred with their aliases: faces, hands, people at work, animals, food, travel, objects and symbols, keycaps. They may be written with or without the colons, in any case.

An emoji that also has a plain text form - a heart, a sun, a check mark - carries the variation selector that asks for the colour picture, so it does not show as a black glyph in one font and a colour one in another.

An unknown shortcode is a strict error that names the nearest one that exists - did you mean ':rocket:'?. The emoji pipe does the replacing for a whole text: {$comment | emoji} turns every shortcode it knows into its emoji and leaves the rest, and a time like 10:30:00, as written.