I think some people watching that video may just wind up getting confused, as it doesn’t show the exact steps clearly and cuts quickly between screens a lot. I mean, I already knew how to do it and I found it confusing. So tl;dw :

To give a thumbs up and/or post a comment on any peertube video from your Mastodon account:

• On the video’s page, copy the url from the address bar
• Go to your mastodon and paste it into your Search box
• Click “Posts matching <the url you pasted>”
• It will show the post and you can then Star it to “Like” and/or Reply to it to post your comment, both of which will show up on their video on peertube, and if someone replies to your comment you’ll get a notification.

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To Follow someone’s peertube channel from your Mastodon account:

• go to their channel page and click the icon by their name (it looks like 2 overlapping squares and has a tooltip that says “copy channel handle”).
• Go to your mastodon and paste it into the Search box, and then click “Go to profile <the thing you pasted>”
• (you can also copy the URL and paste that in mastodon Search, then click either “Go to profile” or “Profiles matching”, whichever comes up)
• You’ll then see their profile that looks just like any Mastodon profile and you can click Follow.
• From then on you’ll see any new video they post in your feed as a toot, and you can Star and/or reply to that toot, which will appear on their video’s page as a like or comment.