"Into" and "onto" are just "in + to" and "on + to" written as one word. If a sentences uses "in" or "on" and you add "to" to the sentence, they combine to become "into" or "onto". E.g. "I'm walking in." - As usual, you can use "to" to indicate direction: "I'm walking into the room." If you just said "I'm walking in the room", it would mean that you're just walking in the room, without going in or out. Adding "to" specifies that you're starting outside the room and walking in(to it).