"Who do you see?"
"Whom" is a dying word in modern English and non-native English speakers (like this course's creators) often have never heard of it, or don't know what it's used for.
I think that course creators knew about "whom" but "who" is nowadays used much more often.
I won't know why people downvoted your comment. What you said is true that "whom" is a somewhat archaic word, just like the impersonal pronoun "one". Native English speakers generally know what it means, but it sounds almost too formal for most usage.
"widzieć" takes Accusative, this is the Accusative form of "kto".
Actually this can be visible in English as well, because many users still would say "Whom do you see" - which is one of the few examples of Old English cases that are still present in English.
You use "czy" in yes/no questions. When English has an actual question word like "who" - you use it's Polish equivalent , and the rest of sentence is like positive sentence.