How do i add a clickable link to a YouTube video and do they make any difference to SEO?

just add your link with "h t t p" in description
if you mean on the video itself, then i'm sorry, that's impossible , unless you buy youtube itself and recode it ; )

Yeah i can testify that you can only add a link to the description. Did the research myself a while back about the possibility of being able to add a link to videos as well, quickly became apparent it isn't possible!