It is actually a thing, called greeklish. It used to be more common/popular during the earlier days of computing/internet due to limited support of the Greek alphabet. It remained popular even later on and even today but i feel it is getting less popular nowadays.
Google translate supports greeklish, you can select Greek and type greeklish and it will work.
Afaik something similar is popular in Arabic that includes digits because they look similar to the Arabic letter and using digraphs (like "dh") can be ambiguous when these letters really come next to each other
It is only using latin letters and it is 1 to 1 for the most part. But it isnt defined somewhere, there are different variations for some letters and the Greek alphabet is slightly different than the Latin one.
For example the letter Greek letter "χ"(similar sound to English "h") can be written as "x" or "h". Χ looks like x but sounds like h, so either works.