What SaveToMP4 does
SaveToMP4 is a free online YouTube converter. You give it a public YouTube link and it hands back a normal file: an MP4 video at up to 1080p, or an MP3 at up to 320 kbps. The file is yours to keep offline, so it plays on a plane, on a train, or anywhere the signal drops.
It is a web page, not a program. That matters on phones, where installing a downloader app is either impossible or risky, and on work or school computers where you can't install anything at all.