"Language" vs "Dialect" is a matter of degree. If you have a tiny bit of exposure to, say, German or Icelandic, you can actually catch a fair amount of what's said in those languages given the context. The same is true for Mandarin and Cantonese. A lot of the words really are just different pronunciations of the same word; 我 (me) is wo3 in Mandarin, and ngo5 in Cantonese; 你 (you) is ni3 in Mandarin and nei2 in Cantonese. A lot of the grammar concepts are the same; you don't conjugate verbs, you instead have "markers" that indicate things like "completed action" or "experienced action"; measure words, and so on.
But a lot of the grammar and words are just different -- at least in the spoken language; and typically subtitles are all in Mandarin.
So my son watches Peppa Pig in Cantonese; I can read most of the subtitles in Mandarin. Peppa will say, "Da4 di, nei2 tai2 ha2!" (Look, Daddy!); but the subtitles will say 爸爸，你看一下！ (ba1 ba! ni3 kan4 yi2 xia4!) Note that only one of those four words is the same (你/ni3/nei2). "da di" (Cantonese) has been replaced with "baba" (Mandarin); "tai2" is replaced with "kan4" (a different character); and "ha2" has been replaced with "yi2 xia4" (an extra word).
"It's all Chinese" is a sort of fiction; and from my outsider's perspective, a fiction which heavily favors Beijing and the Han majority at the expense of the various minority groups. Written Mandarin is pretty close to spoken Mandarin; "official" written Cantonese is very different than spoken Cantonese -- to the extent that the verb "to be" is a completely separate word.
But a lot of the grammar and words are just different -- at least in the spoken language; and typically subtitles are all in Mandarin.
So my son watches Peppa Pig in Cantonese; I can read most of the subtitles in Mandarin. Peppa will say, "Da4 di, nei2 tai2 ha2!" (Look, Daddy!); but the subtitles will say 爸爸，你看一下！ (ba1 ba! ni3 kan4 yi2 xia4!) Note that only one of those four words is the same (你/ni3/nei2). "da di" (Cantonese) has been replaced with "baba" (Mandarin); "tai2" is replaced with "kan4" (a different character); and "ha2" has been replaced with "yi2 xia4" (an extra word).
"It's all Chinese" is a sort of fiction; and from my outsider's perspective, a fiction which heavily favors Beijing and the Han majority at the expense of the various minority groups. Written Mandarin is pretty close to spoken Mandarin; "official" written Cantonese is very different than spoken Cantonese -- to the extent that the verb "to be" is a completely separate word.