Everything happens in your browser
When you open a video, your browser reads it from your disk into the page. The tool works on it there and hands you the new file as a download. At no point is the video sent anywhere: the site has no upload endpoint.
The browser's own video engine
Modern browsers come with WebCodecs, the same video decoders and encoders they use for video calls, often running on your device's graphics chip. Scissor Film reads the video's container (MP4, MOV, WebM or MKV) with Mediabunny, an open-source library served from this site, decodes the frames with WebCodecs, changes them (cuts, crops, turns or resizes them) and encodes them again, usually as H.264 with AAC sound in MP4, or VP9 with Opus in WebM.
Without re-encoding, where possible
Some jobs need no new encoding at all. Removing the sound, changing the container from MP4 to MOV, or cutting at a keyframe copies the compressed video as it is, so it is fast and loses nothing. The tools do this whenever they can.
The editor and the screen recorder
The video editor shows your clips with the browser's player and keeps the project only in the open tab. The screen recorder captures your screen with your permission and records it with the browser's own recorder, on your device. Subtitles from speech use the Whisper model, downloaded once from this site and run in your browser.
Check it yourself
Open your browser's developer tools, choose the Network tab and use any tool. You will see the page and its scripts load from film.scissor.studio, and nothing go out when you open or save a video. You can also turn off your connection after the page has loaded: the tools keep working.