Nothing is uploaded. The file is read from disk, FFmpeg runs in WebAssembly, and the result is handed back as a blob. The engine is about 10 MB compressed, fetched once and cached. The core is single-threaded because the threaded build needs COOP/COEP headers, so a 10-second 720p clip takes a few seconds rather than a fraction of one. No size caps, no queue, no account.
Two optional server features: "Get a link" hosts a finished file if you want to paste it in a forum, and a WhatsApp sticker page can deliver a sticker to your phone. Both are off until you press the button.
The glue code on the page is plain JS; the FFmpeg build is the unmodified @ffmpeg/core 0.12.6 (GPL, libx264 and libx265 included). I would like to hear where it breaks, especially on Safari, and which tool you are missing.