I m searaching package for my OpenWrt 18.06 so I can download a hole playlist from Youtube.
I am pretty sure such a thing does not exist for several reasons. Unless you have a USB drive or something connected to your router, or you are using OpenWrt on say an x86 machine, you wouldn't have enough storage space anyway. But fundamentally, Youtube is a streaming service and doesn't let you just download the content (the exception being if you are a Youtube Premium subscriber, you can download the videos to your mobile device using the YouTube app - this article explains that feature).
Another question is why your router should download that stuff...?
Makes only sense if you have a very slow internet connection.
But give this one a try if you familiar with the shell and know how to install python and pip on openwrt.
I don't know if such thing exists or not. There are not torrent clients, so this shouldn't be different as far as storage is concerned.
There are in fact web sites and desktop applications that let you download YouTube videos, and some applications allow you to download playlists. So it's possible. Whether it's legal or not, that's another story.