i can see how you can find information ON a webpage, but when looking for information WITHIN a website, shouldn't you find information IN the website?

I must confess that I have never thought about it before. We usually say, for example, the information is available AT grammar-monster.com, rather than on or in grammar-monster.com, but I am happy to be corrected.

Generally speaking, "on" aligns well with the idea of "on top of" and "in" aligns well with the idea of "inside". You have to be quite flexible when thinking about this. E.g., "on a plate" and "in a pie" are obvious, but a word being "on a page" but "in a sentence" are a little less obvious. However, it doesn't always work out that way. Certain words attract on and some attract in for no real good reason. Sometimes, you just have to know (through common usage). I feel that "website" attracts "on" every time.

Yes, good point. When you use the URL (e.g. www.grammar-monster.com), "at" is definitely the right word.