A lot of them are shorthand for phrases that exist in other programming languages. If you watch the whole video you can see him type things like match to get the ≅ or something like that I don't know I didn't follow at all It's pretty heady shit.

Yeah, just watching and learning about their existence now, but it definitely looks like people took the full ASCII library and decided to use it for all common operators and operations in the language. Really cool, but yeah, kinda arcane until you learn the characters.