Content with a certain age threshold triggers login. The last time I looked at this, embedding these videos was still possible without logging in. So there are definitely ways in accessing the content without authentication.
Hm. If embedding works maybe my ad-blocking is sufficient; or I just haven't come across any that require it. I mostly just watch woodworkers/machinists/electronics/etc. Sort of conceivable it could be age restricted but would also be surprising.
There's also members-only content on some channels that requires a paid subscription to access.