I’ll start by plugging Harvard’s free courses catalog as well as Udemy
Edit: Gonna add 2 more I remembered-
Blender - I wish I had more time to learn it, but I did start the infamous “Donut Tutorial” once!
Watch Cartoons Online - Lots of good older stuff!
It’s Plex but free and without a central login server handled by a third party
It’s also got a few fewer/not as functional features and no live TV (whoopty do?)
The Arr Suite are what you’re looking for to find content, works with either Plex or Jelly in (or others)