TikTok supports native emojis on all platforms which can be inserted with the system-wide emoji keyboard. In addition to native emoji support, TikTok has its own emoticon set which can be accessed via the inline smiley face icon in comment fields.
These custom emoticons in TikTok use text-replacement strings, whereby a shortname is inserted, wrapped in square brackets i.e.: [loveface] inserts a custom emoticon which looks similar to the 😍 Smiling Face With Heart-Eyes emoji.
When posting a comment on TikTok, each shortcode is replaced with a TikTok graphic.