YouTube videos seem to have small movie clips a lot these days..

I mean like this guy https://youtu.be/u7ga6BGR1e4. Is there a database of clips somewhere where you can buy them to add them to your videos? Thank you for your help.

maybe somewhere like adobe stock?

No they just seem to be general videos. I mean like clips from actual movies.

Can anyone advise?

I'm guessing that big channels like WatchMojo and Screen Rant that use movie clips in practically all of their videos probably have their own local databases of clips, don't forget these channels have lots of staff some of whom will be employed specifically to source clips they need. If you have a small channel that's not in an MNC I wouldn't risk the copyright issues.

The scenes in those videos aren't necessarily clips from movies. In fact, both of them are from TV shows. Specifically though, they're famous memes: here and here. To someone up to date with internet culture and therefore knowledgeable about memes, it's just a matter of being able to recall the right one for the moment, and I guarantee you that's what's going on here. There's no database - the creator thinks of a meme he knows of that would go perfectly here, finds the corresponding clip on YouTube, of which there are many, downloads the video and incorporates it into his clip (afaik there's no copyright infringement because of the shortness of the clip - I think under 30 seconds is legal). YouTube content creation is generally a young person's game for this reason.