Don't know about iOS, on Firefox (Linux and Android) uBlock Origin works. On Android you can also use Newpipe [1] for an ad-free Youtube experience. There is another app called Musicpiped [2] which plays ad-free audio-only versions of Youtube content, as the name implies this is mostly meant for music but it can also be used for lectures etc.

[1] https://newpipe.schabi.org/

[2] https://f-droid.org/en/packages/deep.ryd.rydplayer/