Do Downvotes on YouTube do anything?
The only thing it really does is tally the amount of likes and dislikes a video gets and display it to the public. The YouTube creator can choose to hide the amount of likes and dislikes, but they rarely choose to do so.
Can Youtubers see Downvotes?
There is no way to see who liked your YouTube comment, and likewise there is no way to see who gave you a downvote. YouTube keeps these comment likes or dislikes private for the safety and security of users, but it's likely a safe bet anyone who left a positive comment on your comment also liked it.