@Shuffletime
Tbf he's only slightly incorrect. Cauliflower and broccoli are the same species. As are cabbage and Brussel sprouts.
Kinda like different breeds of dog, same species, same basic DNA but different characteristics.
Kale, kohl rabi, collard greens, all the different fancy types of cabbages/cauliflowers/kales/broccolis and wild mustard too! All of them are cultivars of the same species. As
Shuffletime says, like dog and cat breeds, they were bred by humans to select for different attributes but they are actually all the same species. So if he calls them all
Brassica oleracea, he will be correct
So, in theory, you could collect seeds from a cauliflower plant and get broccoli if they have been cross-pollinated. In reality, you'll probably get something that isn't very tasty with attributes of both. If you want to get cauliflower seed that produces cauliflowers, they have to be taken from cauliflowers grown in isolation.
Brassica rapa, another species in the same genus, includes the cultivars rapeseed/canola, pak choi, mizuna, Napa (or Chinese) cabbage, tatsoi and... turnips!
Apples are the same, all the different types of apple, eg Granny Smith and Braeburn, are cultivars of the same species. Apple cultivars are grown by taking cuttings, not from seed, so every Granny Smith apple tree is actually a clone of the original Granny Smith tree. If you grow a seed from a Granny Smith apple, you won't get a Granny Smith apple tree.