I'm fairly new to cryptocurrency but i've invested in bitcoins years ago but never understood it.
I guess that by creating this thread after these years, you've probably sold them. Otherwise, you'd have been interested on understanding it earlier.
What would you recommend?
It depends on what exactly you want to learn. If you're interested in the way the crypto-market works, then you should read books about trading. Many of them are free, but there are also some masterpieces on amazon.
If you want to understand the technical background of bitcoin, then I couldn't suggest you a better website than:
Learnmeabitcoin.com. It explains it perfectly, better than any other site. Needless to say that YouTube videos aren't helpful at all.