Many job and government sites cap file size. Enter the maximum size and the tool adjusts the image for you.
With a very small target the image may get blurrier, because quality and resolution are reduced. If the photo must stay sharp, such as a document with small text, set a higher limit.
Always JPG.
Complex images are hard to shrink. Try a higher limit or a smaller original.
Yes. It runs in the browser on iPhone and Android, with no app to install.
Nowhere. Files are processed only in your browser, are never uploaded to a server, and are gone when you close the page.