CORE4D has got a great beginning series:
My Scene Nodes 101 series isn't exhaustive but has some simpler examples:
For more advanced training, Dominik Ruckli has some great videos:
Dominik Ruckli - YouTube
Additionally - once you've got a handle on Scene Nodes data structures and beginning/intermediate setups, you can generally follow along with Houdini / Geo Nodes tutorials. For those, it is hard to do better than Entagma:
Entagma - YouTube