You can't in general use unicode to add decoration like bold and underline to alphanumeric characters. What YayText does (based on a quick look) is generate homoglyphs - characters that happen to be bold or whatever and have a similar appearance. It's a party trick - like generating upside-down or mirror characters.

Homoglyphs are used in 'homoglyph attacks' - to fool people that a URL or email address is genuine by substituting similar characters, eg
mybаnk.com instead of
mybank.com

BBCode works with forums that support it - they still exist. It's a simplified form of html and is turned into real html when a post is displayed.