Where does the Asian dish "sang choi bow" come from?

Probably Hong Kong.

It is written in Chinese as "???", which in Pinyin is "sh?ngcài b?o", noticeably different to any of its usual spellings in English.
However, in Cantonese it is saang1 coi3 baau1. Much closer to the English spellings and pronunciation. Cantonese is mainly spoken in Hong Kong, a great culinary exporter and major influence on "Chinese food" in the west.

?? means "lettuce" and ? means "wrap", etc.

It does in fact have a Wikipedia article, but only on the Chinese Wikipedia. The article is very short and Google Translate does not handle it well. I will include the Chinese text here in case any readers who can read Chinese can find something of the dish's origin or history:

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Of course it's still possible the dish originated outside Hong Kong, the name is very literal and isn't necessarily the original name of the dish. But it is clear it came to the English speaking world by way of Hong Kong.

I can't give you exact origin of sang choi bow but it is certainly not from Thailand or any of its neighbouring countries. I found an article, the author has Chinese/Cantonese Singapore origin and she said she never taste similar dish back in her home country either.

In Australia sang choi bow usually made from minced pork or chicken, stir fried with onion, shallot and various other vegetables, seasoned with soy sauce, oyster sauce and served with lettuce leaves. It is a common option for entrée (appetizer) in Thai, Veitnamese or Chinese restaurants. The dish also served as a second course for pecking duck (made from left-over meat). Different restaurant would have different variation of the dish. The only thing they have in common is, it will always come with lettuce.

I could remember having similar dish from Chinese restaurants in Bangkok. It was also prepared out of left-over meat from peking duck but they never served it with lettuce.

Sang choi bow is waaaaaay different to larb though. Larb is more or less a salad. In fact larb beef in Australia is usually called Thai beef salad, although restaurants normally use sliced grilled beef instead of cooked minced meat.

Larb also seasoned with fish sauce, chilli, and lime juice, never with soy or oyster sauce.

Laab in Thai language is Verb mean to chop meat. We have Northern Laab and North-eastern Laab which the way to seasoning and taste are difference. From your picture Squid larb is north-eastern style seasoning with dried chili, lime juice, roasted sticky rice, fish sauce, sugar. It's taste is sour come first then salty with a little sweet. It's normally spicy because Thai people put a lot of dried chili in it and they also add herbs: culantro, shallot, spring onion, mint leaf.

Sang Choi Bow was originated from Guangzhou, China. As Sangchoi is lettuce in Chinese, and it pronounced similar like "To get rich" in Chinese.

I was also led to believe it originated in Guangzhou. I moved to Guangzhou in 2019 and to my surprise I could not find it anywhere. I've been to many restaurants here and it is never on the menu. I asked my Cantonese Ayi to make it and she had no idea what it was, she'd never heard of it! I showed her photos and she declared it was not a Cantonese dish. Could it be a uniquely Chinese Australian dish? it's very popular there, is it possible early Cantonese immigrants invented it in Australia?

I have to disagree with all the answers above which say that it doesn't originate in Canton/Guangzhou, or anywhere in Guangdong Province. As a matter of fact, it very likely did. At least I have ordered it in many Cantonese restaurants in Guangdong, but usually not under the name sang choi bowl.

If I recall correctly, usually it's called something like ??? siu chow wong, which translates to something like "emperor stir-fry". It might be served with or without lettuce wraps, with the lettuce being perhaps a more recent addition. So it't probably unsurprising that more elderly Cantonese people have not seen this dish.

Moreover, traditional Cantonese siu chow don't really look like South East Asian larb. The famous Hoishan (Taishan) siu chow, for example, usually contains dried shrimp/Mantis shrimp, cauliflower, Cantonese sausage, leeks etc., so it really looks nothing like the minced meat-based SE Asian larb. This might be another source of confusion.

Source: I'm Cantonese.