Add arrows, boxes and text to an image

Not in the exported file, and that is the point. While you are still on the page, a blur or pixelate area is only an instruction and you can remove it. Once you export, the pixels inside it are overwritten before the file is written, so there is nothing underneath to recover. Arrows, boxes and text are different: they are drawn on top, and the original picture is still under them in the saved file.

Because a tool that blurs by drawing a translucent overlay leaves every original pixel in the file, and people have been identified from images that were 'blurred' that way. This one runs the blur over the pixels that get encoded. A light blur can still leave a face recognisable, though - if the point is that nobody can tell, the redaction tool's solid block removes the most.

Yes, and not as an afterthought. Every tool has an Add button that places a mark at a sensible size in the middle of the image; from there the arrow keys nudge whichever mark is selected, and the list underneath lets you reorder or delete anything. Freehand is the one tool that genuinely needs a pointer, and its Add button places a straight line rather than doing nothing.

No. The marks are stored as shapes, not as pixels, and they are drawn once at the size you export. That means a mark keeps its relative weight through any resize instead of being drawn large and then shrunk into a soft grey smear, which is what happens when a tool flattens the annotation into the image first.

Yes, and it behaves like a real highlighter rather than a sticker. It multiplies rather than blending towards its own colour, so black text under a yellow band stays black and readable. The naive version of this washes the text towards yellow-grey, which is the usual reason a highlighted screenshot looks wrong.

No. The image is decoded, marked up and re-encoded by your own browser on your own device, and a content security policy the browser enforces prevents this page from sending it to another domain. That matters most for the blur tool: the whole point is that the sensitive version stops existing, and uploading it to achieve that would be the opposite.