Does this sentence mean that frogs are odd/interesting animals or that they are actually curious about things? The English sentence can be understood either way.
They're actually curious about things. "Interesting" would be expressed with érdekes.
It's a general statement, which is commonly expressed with the plural word in English. "The frog" works as well, though.
This is mentioned in the notes - English speakers tend to phrase this as plural "frogs..." whereas Hungarians will tend to use singular with "the".
I've heard people say (for example), "The frog is a small animal" as a general statement, too. It might be a bit more formal, like something you'd read in a textbook or hear in a lecture. But I think it works OK to translate it that way.
I don't think "the" is necessary. It sounds better without. Btw, if the sentence wanted to indicate that frogs are curious about things then it is not really true... they are rather shy.
If it's taking about the whole general frog category, why didn't it use "egy", instead of the specific "a"?
That's simply how Hungarian does it. "Egy béka" would just refer to any one frog, but "a béka" is the principle of the frog. The froggery itself, what makes a frog a frog. (Or it can refer to just one certain frog.) Maybe think "the species of the frogs".
Hungarian works with principles a lot, and it's a bit hard to grasp if you're unfamiliar with that. A simpler example would be "Itt alma van" - "There are apples here." Or "There is an apple here." With leaving the article out in this example, you do not care about the actual number of the apples, but just about the fact that this space contains apples.