Drop in a picture and the background comes off. It works out from the picture itself whether it is looking at a plain backdrop or a studio screen and takes the right one out, so there is no mode to choose. What it removes becomes transparent, which means what you save is a PNG you can place on any colour.
No. Everything happens on a canvas inside your browser. There is no server, no account, no upload and no usage limit — open your browser network panel while you use it and you will see no request go out. That also means a photo you would not want to hand to a stranger is safe here.
Every pixel around the edge of the frame is used as a starting point, because a photograph of a thing almost always has the background touching all four sides and the subject touching none of them. If that edge colour turns out to be a strongly saturated green or blue it is treated as a studio screen and removed by hue alone, which is what lets the shadows falling on the screen go with it. Otherwise it is filled inwards by colour and stops at anything that is not background, so a white shirt in the middle of a photo taken against a white wall survives.
No, and that is deliberate. A segmentation model is tens to hundreds of megabytes that every visitor would have to download, and the licences on the good ones restrict commercial use. For plain backdrops, product shots, screenshots and green screens, matching on colour is more accurate than a model anyway, because there is nothing to guess: the background is that colour. For a person against a complicated background it is not, and the brush is the honest answer to that rather than a promise the tool cannot keep.
Edge pixels are a mixture of the subject and the old background, so a cutout from a white backdrop carries white in its outline and shows it the moment you place it on something dark. Clean edge colour undoes that mixture, and shrinking by a pixel removes the worst of it. Both are on by default.
Yes. Drop several pictures in, work through them using the thumbnails, and Save all as ZIP writes every one at its original size with the settings you have chosen.