I saw somewhere (I think it was an earlier lesson), that "det" can be used for "what" sometimes, but there wasn't much about that. I'm not sure if it was as a relative pronoun or something else, so I was hoping someone could expand on that.
Also, when "det" is used to me "what" is it always interchangable with "vad"?
No, but this question can be phrased in two different ways with slightly different constructions. This doesn't mean that they're interchangeable in other uses.
Ser du det jag ser?
Ser du vad jag ser?
In the first one, there is an implied som in there. If it were written out, it'd be "ser du det som jag ser?", roughly "do you see that which I see?".
I felt bad reporting this because I have know idea if it has to do with you (Swedish) people, or if there is some other place I should report it. It was really weird. The whole lesson had garbled English especially in places where there should have been written Swedish. The exercises where you are supposed to speak had english instead of swedish text