Over the last few years, the YouTube creator community has grown closer together and collaborations have become a regular occurrence. To make it easier for you to find out which YouTubers participate in a video, Google is A/B testing a new 'featured in this video' section on Android. In it, you'll find an overview of partaking creators complete with links to their channels and the option to subscribe.
Google uses "a range of signals" about a video to determine which YouTubers appear in it, and the experiment is currently limited to a "diverse group of top searched creators across various categories on YouTube." When Google's algorithms find one of these people in a clip not on published on their own channel, they'll appear in that 'featured in this video' section right below the player. If deemed successful, the test might expand to include more creators in the future.
Currently, many YouTubers resort to listing participating colleagues in the video description, which is an arguably inelegant solution.
The new feature is in line with previous additions to YouTube, such as automatic music credits and suggested products. Since this is a limited A/B test rolling out to a "small percentage of viewers," you might very well not see the change right now. Using the most recent version of the YouTube app could help you get in on the experiment, so be sure to check if you're up to date on the Play Store.