Note: this is a plain-language summary of publicly available terms, not legal advice. YouTube updates its Terms of Service periodically, so check the current version on YouTube's site for the exact wording.
YouTube's Terms of Service generally prohibit downloading, copying, or distributing content from the platform except in two cases: where YouTube itself provides a download or similar functionality, or where the content owner gives explicit written permission. In practice, this means third-party downloading of ordinary videos falls outside what the terms allow.
Using third-party tools to download videos that YouTube has not made downloadable is, under the terms as written, a breach of the agreement between you and YouTube. This is separate from copyright law: even where copyright would allow a use, the platform terms can still prohibit it.
YouTube's remedies for terms violations are account-level: warnings, feature restrictions, or account termination in serious or repeated cases. YouTube has historically focused enforcement on large-scale abuse rather than individual users, but the terms reserve the right to act.
These are two different things. Copyright law is made by governments; the Terms of Service are a contract between you and YouTube. Something can be permitted by one and prohibited by the other. For the copyright side, see is YouTube to MP3 legal? and what you can legally convert.
Disclaimer: general information, not legal advice. Read YouTube's current Terms of Service for the authoritative text.