Chrome autoplays Youtube videos through a whitelist.
For clean installs of Chrome, Google will include a whitelist of 1000 sites where autoplay is enabled. The sites whitelisted are based on sites where Google sees the highest percentage of visitors play[ing] media with sound
. As you browse, the whitelist will grow to match your browsing history.
More info can be found here: Improving Autoplay in 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. As you browse the web, that list changes as Chrome learns and enables autoplay on sites where you play media with sound during most of your visits, and disables it on sites where you don’t.