M4A is refused by some car stereos, older players, editors and upload forms. Ogg is open and royalty-free, which game engines and open-source tools favour. M4A is at home in music and voice memos on Apple devices; OGG is the usual choice for royalty-free audio in games and on the web. 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. OGG is written at 160 kbps unless you choose another bitrate.