For Youtube, it is probably a pre-populated white list
https://blog.google/products/chrome/improving-autoplay-chrome/
If you don’t have browsing history, Chrome allows autoplay for over 1,000 sites where we see that the highest percentage of visitors play media with sound.
For other cases, you can refer to the following rules (for Chrome at least)
https://developers.google.com/web/updates/2017/09/autoplay-policy-changes
Autoplay with sound is allowed if:
User has interacted with the domain (click, tap, etc.).
On desktop, the user’s Media Engagement Index threshold has been crossed, meaning the user has previously played video with sound.
solved How does YouTube work?