what's the best youtube series for learning about SEO, digital marketing etc
The series by Neil Patel
Neil Patel is notorious for ripping off other people's articles and calling them his own.
I can't say I've watched much on Youtube but for SEO articles Brian Dean is good at distilling topics into easy to digest chunks so I'd expect his Youtube channel to be pretty good: https://www.youtube.com/user/backlinko
Less sure what you're looking for on broader digital marketing - are you looking for email marketing, pay per click advertising, marketing automation..?
I've actually started a Udemy course which is really good. It's by Rob Percival who does coding tutorials and an Irish marketing guy. And only £20!