I understand. I’d be vigilant too if i were you.
Newpipe
Newpipe is an Android app that’s basically like YouTube music only all free features. I’ve mainly used for ‘streaming’ (watching/viewing) videos. I’ve been using this for many years now and had no issues with listening to the audios
Several sources says here that Newpipe doesn’t convert the format (When downloading, the app shows what’s available for audio bitrates [audio quality] that YouTube can offer)
https://www.reddit.com/r/NewPipe/comments/q24tw4/extracting_audio_without_loosing_quality/
https://www.reddit.com/r/NewPipe/comments/vl187c/does_newpipe_download_audio_format_m4a_directly/
And if it’s safe…: https://reddit.com/r/NewPipe/comments/or6wqp/is_downloading_from_newpipe_safe/
Youtube-DL
For a more strict, lossless (no transcoding, only the original quality of the file) method of downloading, one can use youtube-dl or the GUI
Readings
https://www.reddit.com/r/DataHoarder/comments/m5kgf8/what_is_the_best_youtube_downloader_lossless/
Download the best quality audio file with youtube-dl - Stack Overflow
And this is a user-friendly app on desktop for that
GitHub - jely2002/youtube-dl-gui: A cross-platform GUI for youtube-dl made in Electron and node.js
Raag android
Raag (only for Android. On F-Droid store [Filled with open source apps] downloads the highest quality available that youtube allows and depending on the uploader, but since it’s a small project i wouldn’t recommend since it is buggy (still usable for me as a tech-superuser) in its current state
Codec Tampering
As support with evidence for what WellBeing said:
Info about ‘conversion’ CAN lead to loss in quality but not necessarily, there’s lots of other things to consider such https://www.reddit.com/r/youtubedl/comments/ulwtlx/i_want_to_download_audio_only_does_youtubedl/
But as WellBeing says, VLC - for simplicity - can work if that’s what you want or prefer to use. https://reddit.com/r/mac/comments/v4w5m6/free_open_source_quality_tool_for_downloading/
Quality
The Official FaQ from the team stated on #25 and #35 that sound quality on device doesn’t matter, it’s merely a carrier for the energy, but if you’re not getting results, then it usually means you have converted it to a different format (which can lead to massive loss in quality) using whatever software that did it
Best lossless, original quality audio - reading
https://www.reddit.com/r/DataHoarder/comments/m5kgf8/what_is_the_best_youtube_downloader_lossless/
If it comes down to ‘lossless’ HiFi (Hi Fidelity) music streaming, YouTube (256kbps for premium users / 128kbps for free users) doesn’t host HD / HQ like Spotify does (360kbps)
More details here
https://www.reddit.com/r/4kdownloadapps/comments/hbcb28/youtube_myth_busted_320kbps_audio_availability/
When audio gets uploaded to YouTube is the codec still intact? The tech folks answer here detailing how it works and how to download the best available quality How to download YouTube videos with no quality loss? - VideoHelp Forum
And for anyone concerned about quality of sound on YouTube vs Spotify (if disregarding the FaQ): https://www.reddit.com/r/spotify/comments/pbal6x/youtube_music_vs_spotify_sound_quality/
TLDR:
- For ‘best results’ / less risk stream as Wellbeing and Sapien said
- For downloading choose the highest bitrate that’s offered/available by YouTube (YoutubeDL + Spotiflyer have those capabilities)
- Don’t use a converter.
- File extension (IDEAL: .opus) (questionable: .m4a, .ogg, .flac)(reading) (Reading2)
- Sound equalizer adjustment to your taste. Read the FaQ