FLAC files are large, and Apple Music, many car stereos and older players will not play them. AAC is more efficient than MP3 at the same bitrate. FLAC is at home in archiving music at full quality; AAC is the usual choice for efficient audio for streaming and video soundtracks. AAC is lossy, so keep the FLAC original if you may need full quality later.
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.