"That girl resembles a boy."
Translation:Cette fille ressemble à un garçon.
I believe ça is a demonstrative pronoun, so it is used in place of a noun, and cette is a demonstrative adjective, so it is used to quantify a noun.
When you say "That girl" you're quantifying "girl", so you say "cette fille". If you just said "That resembles a boy" you are using "that" as a pronoun, so you could say "ça ressemble à un garçon".
Think of ça meaning more like "it", you wouldn't say "It girl resembles a boy"
Cette fille can mean both "this girl" and "that girl." French doesn't make the same distinction as English.
If you really feel the need to differentiate between "this" and "that" you can add -ci or -là to the end of noun which is similar to colloquially adding "here" and "there" (respectively) after a noun.
- Cette fille-ci est une amie à moi = This girl [here] is a friend of mine
- Cette fille-là est une amie à moi = That girl [there] is a friend of mine
It's just a bit of emphasis to differentiate between this / that, but ask yourself: how often would you be legitimately confused by replacing "this" with "that" in English? Really it only comes up when there is a need to differentiate or compare things -- which is when you'll start to see -ci and -là popping up more in French.
My guess would be because "vous" functions as an indirect object pronoun in your example. For example, you could say, "Je ressemble à mon père." (I look like my father.) And then you could also say, "Je lui ressemble." (I look like him.) The indirect object pronoun "lui" is used to replace the words "à mon père," so the "à" is still kind of represented there by the word "lui."
I don't know if this means that it would be wrong to say "Je ressemble à vous," though. I'm not sure if it is another acceptable way to say "I look like you," or not.