Would the meaning of the context changed if I say "What were you doing" instead?

What is "light-years"?

A "light-year" is a unit of measurement used in space that is equivalent to how far light travels in one year, which is extremely far. Therefore, this unit of measurement is used when referring to huge distances in space. One light year is approximately 9.5 trillion kilometers.

Are "toothin' " and "puncha" a short form of something? Is something skipped to these words?

"Tootin'" is a slang term that can be two different things: it could be used to emphasize "tough," or it could be used to make Finn's song sound better. "Tootin'" is a very informal term and is not used often, so I do not recommend using it. "Puncha" here means "punch." "Puncha" is not a word in this sentence; it is just how this character is pronouncing "punch" in this scene. It can sometimes be used to mean "punch you," but since he says "yo" (meaning you) after "puncha," it just means "punch."

What does "go for" mean? And how do you use it?

The phrase "go for", in this context, is used to say that you would like to do or have something. So it is a causal way if showing choice. Here the passenger is saying he would not mind having some " beats" or music during his journey. Ex: A whole outfit for less than $5! I could go for that! Ex: I think I’ll go for the steak.

I wonder whether "baldness" is also a popular meme/topic in the United States. (In my country, many people make a joke about being bald.)

Yeah! It's common in the United States to tease people and make fun of them for being "bald". Some bald people make jokes about it themselves. I'd say it is sensitive in some cases, like if someone loses their hair due to an illness, then people won't joke about that. Ex: My uncle is growing a beard, but he's bald on top, so his head looks upside down! Ex: Nice forehead.

What does "unto" mean?

Nice question! 'Unto' just means 'to'. It's an old-fancy way of saying 'to' and is not commonly used anymore.