To add to Aaron Walton’s answer: chat and YouTube comments use ad hoc romanisations of Greek, which are called Greeklish. Greeklish is kind of unstable, and there are two different families of transliteration, phonetic and orthographic.
Bizarrely, I can’t find a mapping anywhere. FWIW, this is my Greeklish alphabet:
EDIT: Many thanks to Uri Granta for linking to http://eclass.uoa.gr/modules/doc… . It is a very good overview of the sundry ideological issues around Greeklish, including the huge ideological split between phonetic and orthographic transliterations.