You enter regions as page:x,y,w,h in fractions of the page, and KukPDF redraws every page as an image with black rectangles over those regions. The covered content is gone, but the whole document becomes images.
Your file is processed in your browser and is not uploaded to a Kuklabs server. The page does contact Kuklabs to check your plan, without sending the file.
Yes. Each page is rebuilt as an image with the black bars drawn into it, so the underlying text of the page is no longer in the file.
No. Every page becomes an image, not only the redacted ones.
Use page:x,y,w,h with 0 to 1 fractions, for example 1:0.1,0.1,0.4,0.08. Separate several regions with a semicolon.