MP3 is lossy, so editing or re-saving it repeatedly degrades the sound. AAC is more efficient than MP3 at the same bitrate. MP3 is at home in music and podcasts that must play anywhere; AAC is the usual choice for efficient audio for streaming and video soundtracks. Both formats are lossy, so the content is compressed a second time; start from the best copy you have.
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. AAC is written at 192 kbps unless you choose another bitrate.