Ogg Vorbis is common in games, Linux tools and some recorders, but iPhones, iTunes and many hardware players will not play it. MP3 plays on all of them. Both are lossy formats, so the audio is compressed a second time.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine. MP3 is written at 192 kbps unless you choose another bitrate.