A black rectangle laid over text can be copied and pasted: the text is still there, underneath. Here, the page you draw on is rendered as an image and replaces the original. What is covered no longer exists in the file.
What it does not do: A redacted page loses its text: it is no longer selectable, no longer searchable, and it weighs more. That is the price, and there is no shortcut: hiding an area while keeping the page's text would leave the characters in the file, where any editor finds them again. Redaction also does not remove what the other pages say about the same subject, nor the metadata of an attached file.
Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.