The legality of youtube mp3 conversion addresses mostly copyright infringements and platform terms. YouTube being a copyright protected portal, does not allow (either legal/ illegal) YouTube conversions and downloads. YouTube-MP3 (2017 case) These were the legal actions that ended one of the largest conversion sites hosted at youtube-mp3.org, proving to what extent copyright infringement is prosecuted in digital media.
Per YouTube's terms of service, the downloading or distribution of content is not allowed unless it has been predictably made available for such a purpose by the platform (this does allow things like videos you're permitted to download so they may be watched offline in YouTube Premium). The more than 2 billion monthly users on YouTube are subjected to the stringent set of rules intended to safeguard the intellectual property rights of creators and media companies. Breaking these rules could get you in trouble with the law, or even your account suspended.
YouTube channels that bear Creative Commons licenses allow downloads and modifications, but the user must comply with individual license terms. For example, a video may be marked as Creative Commons but still require an attribution or limit commercial use. Technically, converting these videos to MP3s for personal use generally falls within legal bounds but using them commercially without prior permission can and does result in compromises.
For personal use, this is usually considered a legal gray area (downloading for private purposes might not get you into any real trouble). But they could face fines, lawsuits and even be criminally charged if caught redistributing or profiting from the bounty of new music that will soon flood Kazaaland. A 2018 study found that more than 80% of YouTube users had downloaded videos in some fashion, with no regards for the legal implications.
Also, many countries have unique rules for the law — and digital downloads. It might legally be more allowable to have access in some European countries than others. Please be aware of the legal issues in your own jurisdiction regarding copyright.
To be safe, you should only use this type of service on sites like ytmp3mp4. download for content that’s either under a Creative Commons license, or when the file owner has provided authorization to download their dance. This should keep you on the safe side of YouTube's and copyrights' terms gyaro making your youtube mp3 conversions basically legal as well ethical.