Any tool claiming to "convert" YouTube to FLAC is simply taking a compressed lossy source (AAC/Opus) and wrapping it in a larger, "empty" lossless container. You get a much bigger file with zero improvement in sound quality.
If you are looking for the best way to handle high-quality audio in this context, 1. The Reality of YouTube Audio Quality yt flac best
Converting a 256 kbps AAC file into a FLAC file (which supports up to 1,411 kbps) doesn't add missing data; it just creates a much larger file containing the same lower-quality audio Best Use Case: Any tool claiming to "convert" YouTube to FLAC