Using YouTube videos in iMovie

That would be a violation of the YouTube Terms of Service. From subsection "Your Content and Conduct:"

"License to Other Users

You also grant each other user of the Service a worldwide, non-exclusive, royalty-free license to access your Content through the Service, and to use that Content, including to reproduce, distribute, prepare derivative works, display, and perform it, only as enabled by a feature of the Service (such as video playback or embeds). For clarity, this license does not grant any rights or permissions for a user to make use of your Content independent of the Service."

Regardless of any legal situation millions use YT videos in their own movies . . . usually just parts to illustrate a point or make a comment.

If it is purely for private use who is to know?

Commercially you would be on very shaky ground if you did not at least publish the name of the original creator or get permission.

There are many clips (particularly green screen) where the creators expressly give permission for you to download and use.

The whole situation is similar to when everyone had VHS recorders and taped programmes off air. That was illegal strictly speaking but nobody was ever prosecuted to my knowledge.

O.k, and what is your question or issue exactly?

If you have Youtube Premium you can download videos, whether you can then import them to iMovie or not depends on how YouTube handles the downloads on a Mac if it even allows it.

Some cursory research indicates Safari does not support Youtube downloads, but Firefox, Chrome or Edge do.

https://support.google.com/youtube/answer/11977233?hl=en&co=GENIE.Platform%3DDesktop

Do note the owners of said videos may not appreciate their videos being used without permission on a project specially a commercial one.