In the USA, I mean.
The correct question would be to download the videos without the webpage and the ads.
Super illegal. Like 10 years maximum security prison stuff man. /s
No.
I use VLC for this.
No, every time you watch a video you actually download it in chunks. It’s illegal to redistribute the said video or make money off it without permission from the creator. That’s called stealing. Apart from that you are free to do whatever you like, YT ToS might say otherwise but thankfully Google isn’t an authority yet.
Actually U.S. courts make a distinction between “streaming” which is temporary and require a connection for every repeat viewing and “downloading” which stores more permanent version that can be watched without an internet connection.
The YouTube ToS allow you to stream videos. They restrict downloads. If you break that, your “cached stream” isn’t legal to begin with. It’s like going into a store giving free samples and taking a box full. You have to follow their rules, or the legal system may rule against you.
Not that it realistically matters for personal local copies. Who knows, there can be a possible fair use exemptions in certain cases.
Write down a list first of what you want, then start downloading.