I am supporting several interesting YouTubers on Patreon that offer early access and they just post the link to the unlisted YouTube Video to their Patreon Feed.
Technically there is no access control other then that YouTube links are not easily guessable.
It is probably not a huge concern for these YouTubers if there are a few people who see the video early when they are not supposed to.
It's worth noting this isn't the best method of doing it. Some people like to make personal YouTube playlists, and they like to include, or sometimes specifically add Patreon reward unlisted videos to their playlists. They then forget to list their playlist as private, meaning it's discoverable via a youtube search.
Technically there is no access control other then that YouTube links are not easily guessable.
It is probably not a huge concern for these YouTubers if there are a few people who see the video early when they are not supposed to.