Technically watching a video only streams video chunks into memory. But of course there are tools pretending to be a youtube client to stream it to a file.

Technically "download" just means to transfer to a computer/device; doesn't need to imply persistent storage. "Delete" doesn't need to imply from non-volatile memory either; it can be volatile.

The simplest, best, most reliable, less fragile system is just storing the file on disk as it comes. But streaming software bends over backwards to avoid this simple solution for the lawyers. It always re-evolves with each new streaming mechanism.

In fact when youtube started the entire file would be downloaded to /tmp/. And it was this way for a long time. Then the lawyers heard about it and instead youtube would download the file then delete it and read from the filehandle of the deleted file. Then flash went away. And the replacement video streaming services would send chunks that would be on disk and then the lawyer pacification process repeated and now everything is only stored "in ram" but often, and on many systems, that isn't true either. But it's enough to keep the lawyers calm.

There are technical reasons to choose streaming vs on demand download without involving lawyers. I'm sure regular user would prefer to save bandwidth when opening a 1 hour documentary and only watching first minute.

You've misunderstood. It's entirely technically and easily possible to use the current streaming model of only downloading the file bit by bit (as if there is any other way) but instead of wasting RAM and making things more complex just add each bit to the file on disk as it downloads. You've set up a false dichotomy.

to the layman download means the whole video is stored in its entirety on my disk. and possibly to lawyers too. that is not happening with streaming videos. youtube videos in particular most likely never hit the disk. not even the cache. when i am watching youtube then i am not deleting the video after it is done. the "downloaded" segments disappear automatically as i watch. so there is no delete step that i could skip in order to keep the file. rather, actually downloading a video requires an entirely different operation.

"to the layman download means the whole video is stored in its entirety on my disk."

Yes, that's exactly what it means.

No its the same operation just you save instead of not save.

It used to be but not now. Your browser is served little slices of the video and audio, and the player adapts in realtime, giving you lower or higher bitrate, depending on your network congestion.

yt-dlp, as far as I know, always asks for the same consistent quality/bitrate of your choosing (and saves them to disk and joins the segments and muxes the video and audio into one stream)

Bitrate is irrelevant. It’s a download either way. Serving little slices is downloading.

My point it that it's not the same slices firstly, and secondly the muxing of the slices and streams is an extra operation that the browser cannot do.