Youtube-dl doesn't do anything your browser doesn't do already do when accessing said video.

Also, I asked what is being circumvented, not what the tool does.

It boils down to the intention. The intention is to download videos to your computer (as viewable mp4 files for end users) which isn't legal, as they are intended to be streamed (or possibly removed in the future) with ads, stats, analytics, recommended etc.

I definitely don't defend this and I know that this whole copyright and DRM thing is fundamentally flawed by design, but it unfortunately works this way legally.

If it boils down to user intention then you can't also argue the tool is circumventing anything. Either mimicking a browser and working the way google expects any client not written or authorized by them, such as a web browser, to is ok or it's not.

I don't think saving things is in general a copyright violation?

Knowingly and intentionally obtaining unlicensed content is a violation. Unauthorized redistribution is a violation. Saving and copying things isn't (AFAIK) a violation.

Saving things is creating an unauthorized copy, which is fundamentally the basis of copyright. Some territories have explicit allowances for personal copies but many don't.

There are a variety of reasons that you can have a copy of a copyrighted work it a way the copyright holder didn't intend, such as time shifting or media shifting.

And in many juristictions they are explicitly defined, and don't apply to copyrighted work generally, only when acquired under specific circumstances.

Format shifting is explicitly illegal in the UK for example, and timeshifting is restricted to works obtained from a broadcast source by the person performing the copy, and not to an on-demand one, in many territories.

Although the OS might do that as well when you watch a video and put the computer into hibernation mode.