WebTorrent is a free and open source torrent streaming app. It streams videos instantly when given a torrent magnet link. A torrent index you can use 1337x.
Usenet is another good source of video downloads. Newsdemon is a good Usenet provider. You can use Newsdemon with NZB.SU (index) and SabNZBd (downloader). Once your media is downloaded you can use Plex as a media manager.
FreeTube is an open source desktop YouTube player. In addition to streaming videos, you can also download videos using this app.
For streaming in a browser Snopyta and Yewtu.be are YouTube proxies running Invidious — free proxy software for Youtube access. It does not make use of official YouTube APIs, it scrapes the website instead.
NewPipe is an Android app that provides a private front-end to Youtube.