First and foremost Youtube videos are downloadable There are browser extensions, 3rd party websites and more for downloading from youtube. In fact, any video that can be played on a browser via Internet is downloadable.
However, embedding a video directly using a HTML5 video
tag is super easy to download.
Take the e example code from w3schools
<video width="320" height="240" controls>
<source src="movie.mp4" type="video/mp4">
<source src="movie.ogg" type="video/ogg">
Your browser does not support the video tag.
</video>
You can just Right click >Save as the video to download it.
There are other aspects too, which makes this approach of directly embedding video unusable in a service like Youtube. Features like video track selection, used for allowing multiple resolution, streaming the video rather than downloading the whole video before even playing, live streaming are just not possible with this direct approach.
Buffer approach
Using this approach, the video
element just points to a buffer, where data is pushed dynamically using Javascript. Using this approach allows the features mentioned above which are not possible with the direct approach.
Check out this article on medium to understand about building your own media streaming HTML5 player.
If you use this approach, then users of your website can not Right click> Save on your video to download it. They can, however use a browser plugin to do so.