Simple. This small page is using ffmpeg.wasm compiled to WebAssembly so that everything can run in your browser and no file needs to be sent to any server.

When there is no possibility to install conversion software on your machine you will need to retreat to some online pages. There you can upload your video and you won't know if they are using it any further or even store it - maybe they'll just add a watermark. This converter is running in your browser and no data is sent anywhere.

This page uses ffmpeg.wasm a WebAssembly implementation of ffmpeg a tool for editing different video properties. By now it is very slow because it is not natively running on your system but uses the browsers capabilities. This leads to certain limits.