Even though you can convert a YouTube video into a FLAC file, the audio quality will never actually be "lossless" or CD-quality because the source material is already compressed The Reality of YouTube to FLAC The Quality Ceiling: YouTube's audio typically tops out at 256 kbps AAC
For those comfortable with command-line tools, reviewers from stemsplit.io recommend . It is a free, open-source tool that extracts the best available audio stream and uses FFmpeg to losslessly convert it to FLAC. Command: yt-dlp -f ba -x --audio-format flac yt flac best