Others have given you a lot of info here on what it is your brother is referring to.
But what this is reflecting in your brother is that he has fallen into a pipeline of watching these videos or hanging out with people that watch these videos and had some insecurities that he’s dealing with that is making these ideas seem like the answer. It would be a great idea to be supportive of him in whatever other hobbies/activities he has outside of watching those videos and spending time with him. Talk with him about anything other than the content of those videos so that those thoughts aren’t even entering his mind. Introduce him to your friends that are also supportive in something other than those videos (you don’t mention your ages, so it’s unclear if you know the same people). Having supportive people around his activities and hobbies will help him lose some of those insecurities that made him gravitate to those videos.
If that fails, what I’ve seen others say works is if says he’s an alpha male say back “I’m not into that furry stuff, but that’s really cool your so open with your sexuality like that, and accepting of others’ sexualities like recognizing the beta males”
This is an age group that does need guidance through, and it’s looking for it, and if not provided with a positive role model will latch on to any that they can find.
As an example, look at “Gregory’s Garden Goofballs” episode from Abbott Elementary. A group of the older kids start hanging out in Gregory’s classroom because he’s the “cool teacher” and he gets uncomfortable with it, especially when they start asking him advice about girls and because he doesn’t have his free time anymore. But then he realizes this can be a good thing and that if they are hanging out with him and he’s giving them advice, at least they are with a positive role model. So he sacrifices his free time to be the positive force for the older kids that aren’t even his students, because otherwise they might gravitate to a less positive source.