Here's the YouTube TOS. I believe they ask you to agree to it along with the cookie policy banner that shows up when you don't have cookies stored:

https://www.youtube.com/t/terms

Edit: actually you're right they don't explicitly ask you to agree to the TOS, but the TOS contains this paragraph.

> If you do not understand the Agreement, or do not accept any part of it, then you may not use the Service

So I guess the agreement is implicit rather than explicit.