Start with a file from your camera, editor or an official export. A link to a streaming site is not that file. This site does not fetch platform videos on your behalf.
Choose a format, then start. The WASM engine downloads only at that moment. Progress is shown when ffmpeg reports it. You can cancel. Large files need memory on the device, not on a conversion server.
WAV is the bulky, simple result. MP3 and AAC are smaller and lossy. If the container uses a codec the engine cannot read, the conversion fails with an error instead of a fake file.